All too often, an adventurer manages to make it to the last floor of a dungeon to retrieve the Holy Talisman of Power... but finds that he has to fight not one, but two bosses at the same time. He has just come face to face with a Dual Boss.
A Dual Boss is a pair (or group) of bosses that you must fight at the same time (not one at a time). These are two or more fully powered bosses, as opposed to a Flunky Boss who decided to bring his Mooks along for the ride.
Dual Bosses may be identical or complementary: Popular combinations include a Mighty Glacier paired with a Fragile Speedster, a Warrior with a Squishy Wizard, or similar combination of opposites. Character-wise, they are often twins, siblings, lovers, or just partners or identical constructs/vehicles.
Dual Bosses usually start out fairly, alternating attacks every few seconds or hits (with the exception of the occasional Combination Attack) in patterns that give the player opportunities to evade and strike back or making the bosses hit each other. This teamwork disintegrates as their health declines however, and as they Turn Red they begin attacking the player independently — although it's not uncommon for each boss to fight individually straight from the outset.
Defeating one boss often causes the other to Turn Red to maintain pressure on the player. Otherwise, the battle becomes significantly easier once the player has taken one of them out. Although if you're really lucky, they may evenshare one health bar, halving the effort required.
A particularly difficult Dual Boss is almost certain to become Those Two Bosses.
Not to be confused with the Duel Boss. Compare Cognizant Limbs, Wolfpack Boss.
Crazy Hand does get fought separately in Brawl's Boss Rush, however.
In addition to that, in Brawl, if it is 2 Player mode, Dark Link and Dark Samus are both battled at the same time, and you and the other player have to defeat them both.
Duon is a variation on this, as it's two giant robots with completely different movesets...attached to each other at the back.
The third level of Gungage features two giant dog-like creatures, one red and one blue, attacking you at the same time. Their attacks include fire breathing, charge attacks which temporarily stunt you, and summoning fire below you. On top of that, you have to fight on a raised platform, so if you do not watch out you can fall off the edge.
Paper Mario has the Goomba Bros, The Goomba King w/ The Goomba Bros, The Koopa Bros. (a quadruple boss who are also an obvious Teenage Mutant Ninja Turtles parody), and several others. Paper Mario 2 had a trio of the Shadow Sirens, replacing one of their members for a rematch. The penultimate boss was Bowser and Kammy.
Link's Awakening also has dual Dodongo Snakes as mini-bosses in three dungeons.
There is a Japanese Mega Man 2 fangame which is essentially one of these - you fight all eight Robot Masters at once!
Another (English) fangame based on 2 has you playing as the original six robot masters, each one fighting one of the other eight. So what happens to Bubble Man and Flash Man? They both team up against Elec Man in one of the Wily Castle stages.
Speaking of which, Gemini Man of Mega Man 3 is a Dual Boss all by himself. His Star Force counterpart, Gemini Spark, is the same in this regard.
Also in Mega Man Star Force, Acid Ace R and Dread Joker R also qualify as this and That One Boss. The sibling rivalry which defined the originals is absent with the remakes, as Omega-Xis quickly points out before the fight. And yes, you have to fight both of these losers in a row.
In the Boss Rush of Zero 2, "one" Boss fight stands out: Zero fighting against Herculious Anchortus (defeated in the previous game) and his brother Kuwagust Anchus.
And again later, with the baby elves in Zero 3.
Shinobido has at least two unskippable Dual Boss battles, but they're actually very easy, since you have to bring down only one of them to win.
Mega Man ZX Advent had Urgoyle and Argoyle the Shisharoids. Also, Prometheus and Pandora.
In the original Romancing SaGa for the SNES you had both a Sequential Boss and a Dual Boss battles of the Minions of Saruin in the final dungeon, in the remake you would only fight all three at once if you defeated them in the final dungeon and collected the treasures that they guarded.
The original arcade game has a recurring pair of Road Warrior-lookalikes who first appear at the end of Round 2, as well as twin sumo wrestlers at the end of Round 4. The third stage also ends with a battle against a trio of claw-wielding acrobats.
Kelbeross, Jaquio's pet dogs, in the first two NES games.
Great Koganei, the third boss in Ninja Gaiden III for the NES, has the ability to create a duplicate of himself.
The Mission Mode in Ninja Gaiden Black and Sigma featured twin bosses that made an already hard game even harder.
Romulo and Julianne, a parody of...guess. First they are fought in sequential order, then both at the same time. They need to die together to avoid one resurrecting the other.
Scarlet Commander Mograine and High Inquisitor Whitemane in Scarlet Monastery Cathedral. Like above, it starts with one, then another comes, resurrect the other, then you have to fight both at the same time.
Jarien and Sothos, Bonus Boss in Scarlet Stratholme.
Skarvald and Dalronn in Utgarde keep.
Swamp Lord Musel'ek and Claw in Underbog
The Twin Emperor Vek'lor and Vek'nilash
The Eredar Twins, Lady Sacrolash and Grand Warlock Alythess. Both must be fought at once, and killing one twin causes the two to fuse together, with the surviving sister gaining some of the former's powers.
The Crusader's Coliseum raid has 3 separate multi-boss fights, all somewhat different in execution. First you fight not one but TWO Jormungar. They have separate healthpools but if one dies before the other, the surviving one Turns Red and starts doing 50% more damage.
The Crusader's Coliseum is home to two more Dual Boss fights. The Twin Valkyr fight is an Ikaruga style encounter with a black Valkyr and a white Valkyr. There are black and white portals around the ring, and players have to click the portals to change color; you absorb damage of the same color as you but take extra damage from the opposite color. You need to switch colors when each boss does her big attack.
The other is Faction Champions fight. You fight 6 (in 10 man or 10 in 25 man) bosses, each a superpowered version of the playable classes. It's the only PvP style boss fight in the game.
The ogres King Gordok and Cho'Rush the Observer from Dire Maul. Gordok is a hard-hitting melee warrior, while Cho'Rush is a spellcaster (may be a shaman, priest or mage at random).
Valiona and Theralion in Bastion of Twilight. While you never have to melee them both at the same time, one is in the air using his or her ranged powers while you're fighting the other on the ground.
Elemental Ascendant Council also in Bastion of Twilight. Four bosses total. You start off against two of them, then they switch out with the other two when one of the first pair reaches 25% health. Then when one of the second pair reaches 25% health, they combine into the Elemental Monstrosity.
Omnitron Defense Council in Blackwing Descent. The fight consists of four separate golems with a shared health pool. Only two are active at the same time however.
Nefarian makes his return as the last boss in Blackwing Descent and he is joined by the reborn Onyxia for the first phase of the fight.
Conclave of the Winds consists of three separate bosses that must be defeated within a minute of each other.
The Four Horsemen encounter from Naxxramas has you fight four death knights at once; letting them get too close to each other means a wipe for the raid, so two are tanked in separate corners with the tanks switching often, and the other two are tanked by ranged casters.
The Illidari Council is another council fight, with four different bosses that must be fought at the same time. They have a shared health pool.
The first three phases of the Kael'thas fight are examples of this. First, you fight his four advisers, one at a time. Then you fight all the legendary weapons at once. After that, you fight the four advisers again, but this time all at the same time. After they're dead, Kael'thas himself enters the fight and then it's a regular single boss fight.
The High King Maulgar encounter in Gruul's lair puts you against the titular boss and a few other ogre bosses that must be fought simultaneously (three are tanked by real tanks, one is tanked by a mage spellstealing his buff, and the fifth is tanked by other ranged dps, preferably two warlocks or two hunters).
Mannoroth and Varo'then are fought simultaneously in the Well of Eternity instance. Players initially focus on Varo'then while Mannoroth fights Illidan and assists Varo'then by summoning demons and using Fel Firestorm to burn the players. Players then focus on Mannoroth once Varo'then dies.
Warmaster Blackhorn combines this with Flunky Boss. Blackhorn is the leader of a pack of dragon riders, so he and his six Vrykul underlings are each riding a dragon, for seven rider/mount Dual Bosses. The fight starts with fighting his minions, who attack the raid two at a time, so melee players have a Vrykul Dual Boss, while the ranged have a drake Dual Boss; and finishes with Blackhorn and his dragon mount, Goriona as the true Dual Boss.
The Will of the Emperor encounter in Mogu'shan Vaults is a Mook Maker which is defeated by beating the two biggest mooks, Jan-Xi and Qin-Xi, while the machine keeps pumping out lesser mooks.
The Protectors of the Endless in Terrace of Endless Spring are an example of Triple Bosses. As expected, they Turn Red when you kill one of them. There are even bonuses for killing the "hardest" one last.
Baten Kaitos, at the end of one rather frustrating dungeon. The bosses in question are large cat-esque creatures that happen to be fire- and water-element creatures.
Both the original and Baten Kaitos Origins also feature Trio Bosses, in the form of three enemy commanders: Giacomo, Ayme, and Folon in the original, and Valara, Nasca, and Hughes in Origins.
At one point in Skies of Arcadia, the party is split into two groups. The two separate parties navigate a dungeon together, but don't meet up until just before the boss fight, which is appropriately against two bosses. There are a handful of other examples, but those two are the most prominent.
The game does this three times with the Full Moon Shadows Empress and Emperor, Chariot and Justice, and Fortune and Strength. First time the two bosses have similar skills and strategies, just leaning towards magic or physical. The next time the two can fuse together and split apart again. The next time one protects the other until you beat it, the protected one creating a roulette of effects each turn.
Then there's also Jin and Takaya.
Many of the Tartarus guardians come in groups of three identical Shadows.
In The Answer, there's Akihiko and Ken, Junpei and Koromaru, and Yukari and Mitsuru.
The final battle in Kingpin is a shootout against both the Kingpin and his invincible female bodyguard.
At the end of the first game, you have to take on Saturos and Menardi.
You also face them together briefly in the prologue, though in this case it's a Hopeless Boss Fight.
In the sequel, you take on Karst and Agatio on top of Jupiter Lighthouse, then fight them again in Mars Lighthouse.
Dark Dawn follows in its predecessors' footsteps with Blados and Chalis, as well as the Kaocho generals Ku-Tsung and Ku-Embra. Blados and Chalis fight you twice over as a pair; the second time, they're escorted by the Chaos Hound, aka Volechek.
In Soldier of Fortune 2: Double Helix, you fight a Dual Boss battle against The Torturer (who's armed with an M60 heavy machinegun) and Deviant1 (who's up on a balcony with a sniper rifle) inside a prison yard. Both characters do increased damage, so much so that Deviant1 can kill you with a single shot. They also can absorb more bullets than a normal human, although the difference isn't too unrealistic. The game is otherwise devoid of boss fights (except for 2 battles against an enemy helicopter).
The final battle in The Chronicles of Riddick: Escape from Butcher Bay is one of these, as you fight two man-sized mechs that serve as the Big Bad's personal bodyguards.
the recurring foes Adecor and Boccos, who you fight several times, and who also act as tutorials to some of the gameplay aspects. They can actually be tough to beat, since Yuri fights them alone more often than not.
There's also Belius who creates a double during your fight. This is stoppable however, as you can relight the candlesticks scattered around the fighting area to end the illusion.
There's also Tyson and Nan and optional bosses Gauche and Droite... Wow, Vesperiareally likes this trope.
Silent Scope 2 has the ninjas, Sho and Kane, two bosses that each take 10 hits from a sniper rifle, teleport around, move really fast all around the screen, and create illusions of themselves to throw off your aim.
The first game had the somewhat less frustrating Tom & Jerry.
The second to last fight in the arcade game Hippodrome involves a pair of assassins. Appropriately enough, the level is called "The Twin Paradises".
Tokka and Rahzar in both the arcade and SNES versions of Turtles In Time.
Zorn and Thorn from Final Fantasy IX. Once they're defeated the second time they fuse into a boss that is slightly more difficult to kill. And then you're done with the infuriating little creeps for the rest of the game.
Biran and Yenke from Final Fantasy X. Made more difficult than usual because you can only fight them with Kimahri. Still not very hard, especially compared to the boss right after them.
Ormi and Logos in Final Fantasy X-2 each get one solo boss fight but are otherwise fought together pratically every time they appear during the first two chapters of the game. A couple of those times, their employer, Leblanc, joins them to make it a 3-on-3 battle against the three playable characters.
Blue Fang and Red Horn in Dragon Quest VIII. Although they're optional due to being part of a sidequest, they have one dangerous move, where they team up and pummel one of your characters simultaneously. This is especially nasty if the one who initiates it has tension built up. However, it can only be used if they're both alive, so killing one will remove this potential threat, giving you some leeway to take out the other.
In Emerald, you team up with Steven to fight Team Magma Leader Maxie and Admin Tabitha in Mossdeep's Space Center.
Pokémon Diamond and Pearl have Mars and Jupiter in Spear Pillar where you fight alongside your rival. In Platinum, there is also Flint and Volkner in the post-game, where they are fought by the player and the player's rival at the entrance to Sinnoh's Battle Frontier.
Pokémon Black and White has subway bosses Emmet and Ingo. They run the Double/Single Lines (respectively), but in the Multi Lines you and the other player character fight them together.
Pokémon Ranger: Shadows of Almia has a battle just before the Final Boss where you fight the three bosses you just fought separately (Rhyperior, Magmortar, Gallade) at the same time. If you are playing a WIFI Mission where you have to help Darkrainote It was here for a limited time only, and it required beating Darkrai., at the end, when you least expect it... BAM! You fight a second Dual Boss, which is Drapion and Gliscor, the bosses you encountered earlier in the game.
Pokémon Mystery Dungeon: Explorers of Time/Darkness/Sky: The first bosses are a (very weak) Koffing and Zubat at the end of Beach Cave. Then try this again in the second story, where you have to take on the entire guild at the same time. In Explorers of Sky, there is a Dual Boss in the first special episode before fighting Jirachi, where you have to fight Snover, Gligar, and Bagon at the same time while playing as Bidoof.
The end of the first "episode" of Lego Batman features a dual boss fight involving Batman, Robin, the Riddler and Two-Face. It's somewhat reminiscent of Batman Forever.
At the end of Icewind Dale II, the player gets to fight the halfbreed twins Isair and Madae (respectively a high-level sorcerer and cleric). They are aided by a wide variety of pretty competent Elite Mooks
Neverwinter Nights has penultimate boss fight against Corrupted Copper Dragon and Corrupted Silver Dragon.
In Neverwinter Nights 2 You will need to fight 2 Black dragons after you try to strike down Crystal Heart for Nolaloth.
In Mystic Square, the fifth game, we have Yuki and Mai as the stage 4 bosses. Interestingly, after downing one, the other gets a new, much more dangerous moveset and heals to full.
Perfect Cherry Blossom has the Prismriver sisters... kind of. The spellcards where all three are together are much more like having one boss with three hitboxes.
Imperishable Night has a thematic example: At the end of Final A, Kaguya comes to assist Eirin. But it's a survival card and the bosses don't move, so there being two of them makes no difference.
Fairy Wars takes it Up ToThree with you having to face the three bosses of the three stages all at once after you have beaten them! Then the Extra Stage pits you against both of the midbosses at once.
In Ten Desires, Seiga, the stage 4 boss, fights alongside Yoshika, the boss of stage 3, resurrecting her every time she is defeated.
Subverted in stage 4 of Double Dealing Character; it looks like you will have to fight both Benben and Yatsuhashi Tsukumo at first, but then it turns out you only fight one of them (which one depends on your shottype). Played straight in the Extra Stage, where you fight both as the midboss.
In the fangame Touhou Labyrinth, the Hibachi twins hit every note of this trope. One is immune to magical attacks, the other to physical attacks. When one is killed, the other gains significant stat boosts and a new arsenal of party-wiping attacks.
In Touhou fangame Mystical Chain, all boss fights are like this, but it also has a slightly off-kilter example in its fight with Fujiwara no Mokou and Kaguya Houraisan: it's really them fighting each other, and you're just collateral damage.
The Final Boss has you fight the Undead Core, alongside a transformed Sue and Misery. On top of that Misery also summons more creatures for you to kill.
Earlier in the game, a pair of dragons function as a Skippable Boss, as pictured above.
The Dragon Angels in Lunar: The Silver Star are the last obstacle you have to reach before getting Althena's Sword, the last piece needed to complete the Dragon Armor ensemble and tackle the Magic Emperor.
A couple of bosses in , including Masa and Mune (before they do a DBZ-style Fusion Dance), and Azala and the Black Tyranno. The Golem Twins, who each counter with "copycat" attacks, are a more infamous example.
The DS version has the Archeofangs, two very annoying bosses who need to be defeated within mere seconds of each other (or at the same time) or the one alive will revive the other at full HP. Doing this is much harder than say, FFV's Gargoyles, because the two Archeofangs have different stats and weaknesses. To make matters worse, they also drain your MP throughout the fight as well. Fortunately, their attacks tend to be relatively weak.
Mona and Lisa in the first and third games. In the first game, they were merely Palette Swaps of Blaze, but in 3, they had their own sprites and Wonder Twin Powers (and yet they are much easier than in the first game).
In two-player mode, every boss except Mr. X is a dual boss battle.
The second game had robots Particle and Oxygen as the bosses of Stage 7. The 6th boss is basically a repeat of the second and third bosses.
Rise of the Triad has an add-on, appropriately-titled Extreme Rise of the Triad (now available as a free download from 3D Realms), where one of the levels forces the player to fight two copies of the game's second boss. Only one of the two bosses needs to be killed by the player to end the level, but it makes the task much harder than fighting just the one, like in the regular game.
Lufia II: Rise of the Sinistrals has an interesting variant. The boss of the Sword shrine is a pair of Monster Clowns who fight separately but automatically revive when you kill them. Once you've beaten them both once, your party works out they're reviving each other so you split your 4-character party into pairs to fight them both at the same time, played out as 2 consecutive boss battles.
Episode 1 has a battle against Captain Kuro and Don Krieg. Only the former is a real threat, almost raising the fight to That One Boss status.
Episode 2 has a few of them. You fight Red Haired Shanks and Dracule Mihawk at the end of island 2. There is also a bonus Dual Boss on the same island against Rob Lucci and Paulie. Island 3 has a boss fight halfway through against Kaku and Rob Lucci, both in their Devil Fruit forms.
Final Fantasy V: Two Gargoyles guard the location of each sealed tablet; You must kill them at the same time (or at least, one after the other within a short window), otherwise one will revive the other.
Plok has the Bobbins Bros. Later on in the game (during a flashback when you play as Plok's grandpa), you have to fight against THREE of them.
In The Hunt has your submarine fight two snail-like submarine robots as a boss. One always faces right and the other always faces left, but other than that, they share exactly the same attacks. Destroying one of them makes the other become a lot more aggressive.
The unreleased game Chimera Beast had the first boss, two lamprey-like organisms which could only be damaged when their heads were visible.
The Mu bosses in Illusion of Gaia are Jack and Silvana, a pair of married vampires. They battle together, complete with a combination attack. When one dies, the surviving spouse flies into a frenzy and Turns Red. Generally considered That One Boss.
Half-Life 2 ends with a fight against two Combine Gunships. While they are the most powerful enemies in the game, they make it even more obvious that the game doesn't have any kind of actual ending and you fight them only because the designers had no idea what else you could be doing once you reached the main villain who does not have any combat abilities at all.
X-Men Legends has a fight against Avalanche and Sabretooth, which is made stranger by the fact that they share the same voice actor. Beat up Avalanche enough, he retreats and you win the fight. Sabretooth is unkillable - he'll just come back from offscreen. Makes sense with the Healing Factor.
X-Men Legends 2 has Sinister and the brainwashed Beast, who is undefeatable. Sinister will immediately make for his reviving machine, which will recharge Beast's health immediately.
In Sonic 3 And Knuckles, Knuckles fights two sub bosses in Launch Base Zone Act 1 instead of Sonic and Tails just having one. If you play as Sonic and Tails together, technically it's Knuckles fighting you as a dual boss. Also Whisker and Johnny in Sonic Rush Adventure. note However, there is a mission that played said Dual Boss, but although it played like normal, you must only defeat Johnny, because if you defeat Captain Whisker, you will fail the mission.
New Destroyman, two cyborgs made from the severed halves of the original Destroyman. One of them fights Shinobu up close with punches, while the other one fights at a distance with Eye Beams and runs away a lot. Because they have separate health bars and can revive each other if one dies, this fight becomes a very annoying game of chase.
The trick is to completely ignore the cowardly one until you take out the brave one, then wait for him to fly over to revive his other half. It's entirely possible to interrupt the resurrection and kick the crap out of him, and he'll keep trying even as you trounce him.
In BioShock 2, after rescuing all the Little Sisters in a level in you have to deal with a pissed off Big Sister (which is also That One Boss for a while). Near the end of the game you are in the room with just a pane of glass between you and the thing you came for when suddenly two Big Sisters come at you.
Left 4 Dead has multiple Tanks attack you after the rescue vehicle arrives. Survival Mode can also see two (or more Tanks at once, as does the finale of Swamp Fever in 2.
On a lesser scale, any encounter with Hunters could count. They always appear in pairs, carry massive fuel rod guns, and have armor that reflects any shots not placed directly on the weak point.
On the first level of Halo: Reach, you have to fight two Zealots at the same time. This is one of only three encounters with Zealots in the game. On the final mission, you have a Wolfpack Boss fight with the Field Marshal and two or three Zealots.
The first game also has a dual Zealot battle on Two Betrayals, at least on Legendary.
On Korriban, if you decide to go Light-Side completely, then after getting the star map in Naga Sadow's tomb, you may end up fighting both Yuthara and Uthar at the same time, although they can both be easily weakened earlier- or you can persuade one of them to join you on the Light Side...
The Meta Rangers in Viewtiful Joe Double Trouble. First, you fight Ranger Log, then Ranger Digi, then they team up to fight you on their final health bar. When you revisit the fight during the Boss Rush, they fight you together right from the start.
MadWorld has the Masters, Jedi knock-offs that come in an older/younger pair (looking rather like Obi-Wan and Anakin). Their answer to the Force is magnets, so they're tethered to each other for most of the fight and accordingly share a health bar.
In Metamorphic Force, you fight a duo of small cyclops midway through the fourth level.
Before that, there's the optional Bella Sisters in the Ganado camp. You thought Dr. Salvador was bad? Try two female versions at once.
Resident Evil 5 tops that with a two-on-two battle against Albert Wesker and Jill Valentine. Played mostly for fanservice, as Wesker proves to be nigh unkillable and killing Jill results in an instant game over. Your AI partner literally advises you to run and hide, and the fight automatically ends in seven minutes when Wesker runs out of spare time. Damaging Wesker enough causes the battle to end prematurely (which can be achieved by hiding strategically and attacking at certain opportunities) and is an unlockable achievement/trophy.
The second time in God Hand that you encounter the Three Evil Stooges, Felix and Bruce take you on together, with Conchita dropping in and out of the fight from time to time. When you fight them in the arena, it is possible to cause this to happen by triggering more than one of their icons.
In the flash game Epic Battle Fantasy 3, you face a 3 headed dragon. Each head acts as it's own boss, though, with different stats and health bars. When you kill one head, stats are doubled for the two remaining heads, and when you kill one more, the last head has tripled stats.
Godcat from Epic Battle Fantasy 4 has you fight each form seperately, then face both at the same time.
Every time the main characters of Disgaea: Hour of Darkness cross over as bonus battles in other universes, they're usually fought in the order of Flonne -> Etna -> Laharl (with Flonne and Etna acting as backup).
The Ninja Warriors Again has a bastardly hard fight against Phobos and Deimos, a pair of giant silver and gold androids in Stage 6.
The Incubuses in Odium. Actually pretty weak both on their own and in tandem. Also sort-of overlaps with Flunky Boss since there are two monsters with them that can inflict the annoying Harmless Freezing on you.
Dead Rising 2 has the twins, who both come at you armed with katanas and can easily take you down in a few hits, making them That One Boss. It's subverted a tiny bit though, because when you kill one of them, you get a cutscene which shows the remaining twin commit suicide by impaling herself.
Vindictus has numerous examples of this trope, with anywhere from 2 to 5 bosses in a mission (not including mini-bosses). Depending on the mission, they are either identical or complimentary. In most missions, they appear at the same time, usually 2 or 3 of them. The Hoarfrost Hollow mission "Prepare for Counterattack" is a hybrid of this and Sequential Boss — the mission starts out with 1 boss, and 4 more arrive, 1 every 2 minutes, as "reinforcements"; each one higher level than the previous. If they can't be killed fast enough, it's possible to end up fighting all 5 at once (it's pretty typical for solo players to have to fight the last 2 or 3 simultaneously).
In the middle of Mission 3 in both the arcade and NES versions of the first game, the player has to fight against twin clones of Abobo before arriving at the enemy's hideout. Later in Mission 4 (in the arcade version only), there's also a battle against twin clones of the Mission 1 boss (a black Head Swap of Abobo with a Mr. T-style mohawk and beard) before the final boss fight with Willy and his bodyguards (who are all clones of the Mission 2 boss, who was in turn a head swap of the player character).
In the arcade version of Double Dragon II, the player has to fight against twin clones of Burnov (the Mission 1 boss) near the end of Mission 3, and then twin sets of all the previous bosses (Burnov, Abore and Chin) before the boss battle with Willy in Mission 4. The final boss battle will be against two Lee Brother clones if a second player is present (one for each player).
The NES version of Double Dragon II features the twin ninja bosses at the end of Mission 2, as well as the Lee Brother clones from the arcade version in Mission 8. Subverted by the Bolo enemies, who always come in pairs (with one exception in Mission 4) but never fight together (the second Bolo always appear after the first one is defeated).
The Chen Brothers in Super Double Dragon, Ron-Fu and Ron-Pyo, in the end of Mission 4. In the American version though, the player has to fight them separately, one in the upper balcony and the other in the lower balcony. In the Japanese version, both are fought at the same time on the upper balcony.
Hong and Wong, the Twin Tigers, in the end of Mission 3 of Double Dragon Advance. They're loosely based on the Chen Brothers from the SNES version.
Don't forget Jabberwocky and Bandersnatch, the palette swapped versions of Enki and Enlil. Which can be fought as a regular mob not very long after their boss battle.
Final Fantasy XIII-2 has Pacos Amethyst and Pacos Luvulite, who revive each other endlessly, so you have to bring them both near to death before killing either one.
The first phase of the final battle in Marvel vs. Capcom 3 pits you against metallic copies of any two of the following: Doctor Doom, Dormammu, Akuma and Wesker (the second joins about ten seconds into the fight). They share a life meter, so knocking one of them out kills both of them. Hitting both of them deals double damage, so hyper combos with large hitboxes are useful.
The twin dragons Devaria and Givaria in Deathsmiles' Extra Stage.
Both battles with Natia in Bomberman Hero. The first time, she's accompanied by the spider-like robot Cronus (though she mostly just floats around taunting you and providing Collision Damage until Cronus is defeated, making this close to a sequential boss), while the rematch suddenly reveals that she has a twin.
The first battle with "The Betrayers" Devola and Popola in NieR counts, as does part of the second battle.
Also, the first boss battle against Hansel and Gretel.
Earlier in the game, you face off against Todd Ingram's bandmates Envy Adams and Lynette Guycott in a mid-boss battle in Stage 3note this is different from the comics, where Ramona fights Envy and Envy later beats up Lynette when she catches her making out with Todd.
The first boss in Cho Ren Sha 68k is this. One neat thing is that you can use the charged ball attack they use for a one-hit-kill if you make them aim at each other before they fire.
The Goat Sisters, a black and white humanoid goat-imp in Rule of Rose attack you simultaneously in a cramped space, making much of the battle a struggle at staying out of their weapons' range.
The two snake things (Fune and Nahime) early on in Metroid: Other M. Sometimes the tougher enemies can be fought in pairs or threes, acting as miniboss battles. Overlaps with Wolfpack Boss in those cases.
Yogleks & Omulgun in Ys. Only Yogleks can be damaged, and when hit, they switch places.
EarthBound does this with the final boss, though Giygas is invulnerable during the first phase, and Heavily Armed Pokey ditches fighting after that phase ends.
City of Heroes has a few occasions when you can find yourself facing multiple powerful bosses, but a classic example is during the Behaviour Adjustment Facility Trial, where the league must face Siege and Nightstar at the same time after defeating them each separately, and like many examples must be defeated at the same time to prevent regeneration. There are also continually respawning reinforcements. Somewhat subverted however in that the standard strategy has teams fighting them entirely separately, while coordinating the battles to ensure that they go down together.
Spiral Knights: the Roarmulus Twins are a pair of gigantic Gun Puppy turrets fought at the end of the Ironclaw Munitions Factory, invulnerable except when one of them hits the other with a rocket. They also appear in the Ironclaw Shadow Lair as the Red Roarmulus Twins. The Gloaming Wildwoods Shadow Lair ends with a pair of Rabid Snarbolaxes (who have Silkwings to heal them).
Captain America and the Avengers has two sets of these as mid-level bosses appearing after a boss flees: Klaw and Living Laser in Scene 1, and the Controllers in Scene 5.
Demons Souls has the Maneaters. Although the battle starts out with only one of them to deal with, another appears after a fixed portion of the original's health has been whittled away. As if this wasn't bad enough, the arena in which they are encountered also happens to be a narrow rooftop walkway which is exceedingly easy to be pushed or knocked off of.
Dark Souls: The Bell Gargoyles on the roof of the church in Undead Parish. Once you get the first one down to half its health, another one appears that breathes fire along with its halberd and axe-tail attacks, also with half a health bar. It's almost impossible to fight both of them with the area attack fire breath and tremendous physical attacks, so it's pretty much compulsory to summon Knight Solaire to tank their attacks while you hack away at their backs.
Dragon Slayer Ornstein and Executioner Smough are found together as the first major boss in Anor Londo and are generally considered one of the tougher challenges of the game. Ornstein leaps and lunges with a lightning spear at high speeds. Smough stomps around at a slower pace, but his giant hammer smarts more and throws you around the arena. Any strategy other than continuously backing away to keep them both in sight is almost a guaranteed death sentence. Oh, and when you kill one of them, the other absorbs their partner's power and comes back stronger and fully healed: Ornstein becomes a giant with a virtual one-hit-kill move, or, alternatively, Smough gets a big lightning charge for his hammer. Fun.
Earlier on, after attempting to break up a domestic squabble, the husband and wife who had been fighting each other turn and fight the protagonist and Harry together.
Father and Grandpa Andore in Final Fight, who appear exclusively in the second area of the West Side stage in which the player must face both at the same time in a steel cage match. They are essentially stronger versions of the standard Andore enemies, who are giant mooks. To make sure the odds are stack against the player, the game throws a third Andore relative named Uncle Andore if the player has a partner.
The first Golden Axe has the Bad Brothers at the end of the very first stage.
The early 90s arcade title Vendetta had the Rude brothers, a giant and a dwarf. The health bar only applied to the giant. Get that bar to zero and the dwarf would die with his brother even if you didn't hit him once, but if you hit him enough times he'd die, leaving his brother still standing. The bonus stage had the player fight every boss two at a time; three in the case of the Rude brothers.
In Strider, you get to fight against the 3 Kuniang sisters at the same time (and Solo, if you chose to ignore him when he first shows up). The sequel has the Kuniang sisters back, plus Hiryu gets to fight two Researchers who transform into a mutant walrus and wolf respectively.
The Kuniang sisters return once more in the 2014 Strider. First, you fight Pei Pooh by herself. Later, during the battle against Nang Poohnote who replaces Sai Pooh from the original gamesin the Residential District, you have to fight her and Pei at the same time. The third and final battle against them in the industrial district pits you against all three sisters (Pei, Nang, and Tong).
Twisted Metal 4 includes a boss fight against Super Slamm and Super Auger.
The Twisted Metal reboot includes a boss fight against the Brothers Grimm, formed by previously playable monster truck Hammerhead and newcomer Slayer.
Dungeon Crawl has the twin unique elves Dowan and Duvessa. Duvessa focuses on melee combat while her brother slings spells from a distance. If you kill one, the other gets a boost: Duvessa goes berserk and Dowan gets several significantly more dangerous spells.
In Time Crisis II, you fight Wild Dog and Ernesto Diaz at the same time, though the battle ends with Wild Dog being defeated and you later fight Diaz in the final battle. Then in Time Crisis 3, Wild Dog and his younger partner Wild Fang fight you together.
DoDonPachi dai ou jou Death Label puts a cruel twist on this at the end of the first loop by making you fight not one Hibachi, but two of them! They're actually at reduced power, as one by itself is hard enough. At the end of the second loop, you fight the two Hibachis again...this time at full power.