In a commercial for Dungeon Siege 3, two ordinary guys pull swords out of nowhere and start fighting ugly humanoids. They briefly fight back to back here.
Anime & Manga
In the final episode of the second season of Soul EaterMaka and Crona do this when fighting the golem.
Appleseed: Ex Machina has one of this right at the beginning with Deunan being surrounded by a bunch of cyborgs as Briareios comes to the rescue, does a fairly Unorthodox Reload ejecting magazines out of his arms, then the two of them proceed to shoot up the cyborgs, "to ask them", with Deunan showcasing her incredible mobility
This occurs in InuYasha the Movie 3: Swords of an Honorable Ruler. The titular Inuyasha and his brother Sesshoumaru dramatically face off against an overwhelming enemy back to back. for about 3 seconds and a half, then Sesshomaru just plays it dirty and use the path InuYasha just opened to leave him surrounded.
In the two-part Samurai Champloo episode, "Elegy of Entrapment (Verse 1 and 2)", both Jin and Mugen fight a blind jumonji-yari expert with their backs against each other at one point. However, she holds back and lets Mugen kill her, since she lost her reason for living.
Likewise, in Mugen and Jin's first fight, Mugen does a flip over Jin's back that temporarily puts them back-to-back.
Science Ninja Team Gatchaman: Constantly. Most of times it was KenandJoe or Ken andJun who did the back-to-back fight, but it any member of the team could -and did- find in a situation that demanded fighting back-to-back with a teammate.
In Ranma One Half, to defeat the dojo destroyer, Ranma and Akane stand back-to-back and link arms to knock the enemy out. Note that this is only in the anime version; the manga has Ranma take it down with one Offhand Backhand. Ukyo and Ryoga also get a moment of this in the OAV version of the Cave of Lost Love story, when the ghosts compare their teamwork and alliance of convenience to Ranma and Akane'srelationship and promptly decide that Ukyo and Ryoga are the only couple present — ignoring their protests that they are not interested in each other that way at all.
Yuuichi and Mai with swords while fighting demons in Kanon, although Mai was about a thousand times more badass than Yuuichi was.
In Mazinger Z Kouji and Boss often did this when they were fighting Iron Masks; Kouji and Tetsuya also did some fewer times. It looked more impressive than the former instance because Tetsuya is a bigger Badass than Boss, because both youths were riding their respective Humongous Mecha when it happened, and because they are Bash Brothers.
Vash the Stampede and Wolfwood in Trigun, notably in the manga where it doesn't only happen twice, but they flawlessly switch places in the blink of an eye both times to combat the opposite opponent(s) when seen fit.
Inverted in anime episode 14 "Back to Back, a Fight to the Death!" Ichigo forcibly gets newfound rival Ishida to do this to stave off a legion of Hollows while at the same time trying to convince him that they are Not So Different.
The anime season "The New Captain Shusuke Amagai".
The opening title includes Ichigo and Captain Amagai back to back.
In episode 186 Rukia and Rangiku Matsumoto (who had been fighting moments earlier) end up back to back facing off against an attack by ninja mooks.
Toshiro and Rangiku at the beginning of episode 222.
Referenced in the Zanpakuto tales filler arc, with Kuchiki Byakuya and Senbonzakura.
Senbonzakura Master, I will watch your back.
Byakuya That's not necessary.
Anime episode #319. While fighting against the Reigei guarding the Senkaimon gate, Ichigo and Rukia briefly stand back to back facing them.
Sailor Moon: Uranus and Neptune. The other girls also do it sometimes, but it rarely works as well as for the above two.
Subverted in Mai-Otome. Shizuru and Natsuki do this during the attack on Garderobe in episode 17, but since they have no chance of winning without their Robes anyway, Shizuru allows herself to be captured so Natsuki can run away.
Played straight in the finale with Shizuru and Haruka, who engage in a Body-Count Competition to see who can destroy more Slaves.
Ditto for rivals Sho and Kenzan in another 2-on-2 duel in Yu-Gi-Oh! GX.
They're not rivals, but Kenzan and O'Brien do stand back to back against a horde of oncoming duel zombies.
A flashback in Cowboy Bebop shows Spike Spiegel and his arch-nemesis Vicious performing this trick in the past when they were hitmen partners working for the Syndicate.
Animerica sure loves using this trope, by pairing up Kiyo and Kasuse, Kasuse and Makoto, Takuya and Makoto, HeelFaceTurn-ed Kiyone and Ron, but the one of the most notable (and most awesome) was the scene in which Josh and Leon fight against Amshel's Army of Darkness (no, not THATone.)
Near the end, Kirika and Chloe use their signature weapons to take on a score of fighters thus in a jaw-dropping display of Bad Ass.
Kirika and Mirielle also do this in the final episode.
Similar to the Dune example under Literature, the Virgo mobile dolls in Mobile Suit Gundam Wing use use the groups-of-three variation a lot. It's justified, however - while one Virgo alone has enough planetary defensors to only generate a shield in front of them, a squad of three have enough to generate a shield around the squad - and by keeping their backs to one another, it allows the squad to fire in any direction.
Asuna and Setsuna from Mahou Sensei Negima! fight off Chigusa's minions in this fashion while Negi runs ahead to take on the Big Bad. When more minions try to stop Negi, Kaede, Mana and Ku Fei show up, and Ku and Mana fight stock baddies in the same back-to-back fashion while Kaede goes one on one with KotaruInugami.
Later, the previously mentioned Kotaro does this with Negi.
In the Grand Finale of Macross Frontier, Brera and Alto do this in their space-plane robots against the Galaxy task force defending the suborned Vajra queen.
This itself is a Shout Out to the final major battle in the original Super Dimension Fortress Macross where the newly married Max and Millia Jenius (the show's two big aces) did the same during a charge into the heart of the enemy formation.
In Jo Jos Bizarre Adventure, Polnareff and Iggy do this. Quite a variation, as they're only fighting one enemy (albeit an invisible insta-killing one) and one of them is a dog.
In an odd example of this trope, Franky, a man who turned his front half into a heavily armored cyborg but couldn't reach his back, has developed a powerful defensive technique that he calls the 'Franky Invincible', in which he goes 'back-to-back' with the floor he's standing on by simply lying down on his back, preventing the enemy from reaching his weak point.
Another odd example happens during Zoro and Ryuuma's duel. The two swordsmen each pull out moves that end up missing and they suddenly find themselves standing back to back. After a moment's awkward pause, they go right back to fighting each other.
Even stranger is Luffy and Buggy in Impel Down. Followed by the somewhat more conventional Luffy and Mr. 2 Bon Clay.
Also, in a straight example, Luffy and Ace in chapter 571.
It's also averted numerous times with Zoro and Sanji, who should be back-to-back True Companions but can't stand each other and end up fighting each other instead. Sometimes while surrounded by an overwhelmingly large force of enemies. Of course, when they do work together, their teamwork turns them into a nigh unstoppable force. Case-in-point: the Davy Back Fight.
Zoro: Hey, cook. We'll work together for 10 seconds.
Sanji: I can deal with that much.
In a cover page, we have Whitebeard and Roger standing back to back. What makes it particulary sad is that both of them are dead.
Usopp, Nami, and Brook pull this off pretty nicely in a three way version when they're fighting off the palace guards on Fishman Island.
In Gintama, Zura Katsura and Gintoki are shown doing this in what appears to be a Last Stand against the Amanto aliens in a flashback. They do it again in the Benizakura Arc, when they are fighting against space pirates hired by Takasugi.
Being Badass magical girls who can kick ass while wearing the frilliest of skirts, just about anyone from the Pretty Cure franchise is shown to pull off moments like this, no matter how many is on each team!
Guilty Crown: Shu and Ayase strike this pose after taking down a few intruders.
During a surprise nighttime training exercise in Maiden Rose, Taki and Klaus end up the only students still not defeated. The moment they decide to take on all the attackers together they pull out their knives and stand back-to-back, laying the foundation for their future Battle Couple status.
Snake-Eyes and Storm Shadow on the cover of G.I. Joe #96.
Hell, Snake-Eyes and Storm Shadow during the G.I. Joe story Hush Job. Although subverted in that rather than actually engage their opponents whilst back to back, they simultaneously somersaulted over the heads of their opponents and then just blew them up with a demolition charge.
In Thunderbolts, the Thunderbolts and Spider-Man were together when a villain tried to take over their minds. Only Mach-1 and Spider-Man fought off the control, and had to fight as Back To Back Badasses. (A severe shock for Mach-1, who found that after that, he could not frame Spider-Man for murder.)
In A Growing Affection, Naruto and Hinata develop an advanced version of the Protection of the 8 Trigrams 64 Palms, where they stand back to back, Naruto covering Hinata's blind spot and providing the chakra for the technique.
The ending of Mr. and Mrs. Smith is one extended back-to-back scene (and, at points, front-to-front.)
Also any fight scene without guns in the Rush Hour films.
In Aliens, the two smartgunners do this in the hive.
A version of this occurs with Seraph, Trinity, and Morpheus going through the Merovingian's club toward his table, back to back, all three with a gun in each hand. They didn't actually do any fighting, but no one tried to stop them.
This also happens with fighting, in the "reality" of the movie, during the battle at the docking bay. After many Sentinels managed to break through the barrage of machine gun fire, the APU users were forced to stop their barrage fire and defend themselves in all directions, with three of them going back to back on each other covering 120° in front of them (covering 360° this way). The scene also involved a dramatic camera movement showing them from above being surrounded by enemies.
In the aptly-named Back To Back, ex-cop Malone and Yakuza man Koji team up to fight off a small horde of thugs.
Subverted (kind of) in the film Stuck on You. Walt and Bob get into a barfight and use this method to defeat a much larger group in a decidedly badass action scene. Of course, they're conjoined twins attached at the hip, so they were not truly back-to-back and weren't really given a choice about how to fight.
In the movie Dog Soldiers, Spoon and Joe share a moment like this, keeping each other covered as they pepper the werewolves attacking the house with machine gun fire.
Utilised by Mace Windu and Obi-Wan Kenobi in Star Wars Episode II.
The Lord of the Rings: Gimli and Aragorn at the gate into Helm's Deep. For several minutes at least they hold off the entire Uruk-Hai army.
Also done by the party in Moria, when they were surrounded by hordes of orc/goblin things ... until the Balrog showed up and scared them off.
Subverted/Lampshaded in Blankman. Eager superhero wannabe Blankman (Damon Wayans) and his brother/reluctant sidekick Kevin/"Other Guy" (David Alan Grier) finally get on the same page regarding crime fighting, and have some generic henchmen to defeat. Blankman dramatically announces "Back to back!" They take up the standard pose, and Blankman is immediately subdued, complete with a girlish squeak, while Other Guy competently dispatches opponent after opponent.
Which is interesting, since spells are normally ranged weaponry and you can only block a certain range of things, so if your block fails or you know you don't have one that will work, you have to duck—and if you dodge without really good communication skills, your partner is going to get it in the back. Always true, but the variety of attack types in Harry Potter makes it a risky thing to do.
Denzel Washington and Mark Wahlberg in the climactic fight in 2 Guns.
Related: This was originally a planned feature of the Return Of The King video game adaptation.
Conan does a one-man version: he can fight off any number of anything, provided he has his back to a wall so he can't be snuck up on.
In R.A. Salvatore's Drizzt Do'Urden novels, this happens at least twice with Drizzt and The Rival Artemis Entreri. Being two of the finest swordsmen on the face of Toril, they handily kick ass doing it.
Dune has this with the Sardaukar, who are trained to fight in groups of three in a formation so that none of them has their back to the enemy. So, it's Back to Back to Back Badasses.
Terry Pratchett's Discworld books features fighting back to back as a part of Ankh-Morpork pub brawls (which has become a team game rather like American football). For the most points, two people have to back up against one another, turn around, look surprised at meeting an ally, then proceed to attack an enemy behind their ally. This is known as a "Double Andrew."
In William King's Warhammer 40,000: Space Wolf novel Ragnar's Claw, when Sven charges into a horde of tyrannids, Ragnar charges after, and they fight back to back.
In Lee Lightner's Sons of Fenris, in the jungle, Nathaniel and Elijah use this tactic.
Star Trek: The Next Generation original novel Debtor's Planet by W. R. Thompson may be the only claim of a Klingon metaphor "to live back to back", meaning to double-up with an trusted partner on alertness during an longer-lasting crisis.
Mace Windu and his Padawan Depa Billaba had a frequently used maneuver that involved this, with one going high and the other low. Even years after she had become a Master and gone off on her own, they were still able to work this in perfect harmony.
Happens sometimes in Redwall. The Guosim crank it Up to Eleven in Outcast with the Guosim Windmill, in which dozens of them form a circle which works in roughly the same way.
In Graham McNeill's Ultramarines novel Warriors of Ultramar a lowly defence trooper - not even a member of the Imperial Guard - with a chainsword does this with an Ultramarines Space Marine Captain against a horde of Tyranids. He does pretty well, considering. Badass Normal indeed.
In Chris Roberson's Blood Ravens novel Dawn of War II, Aramus and Thaddeus at the end just before The Cavalry arrives.
In C. S. Goto's Dawn of War: Ascension, at the Blood Rites, aspirants form bands for this, but many are stabbed In the Back by others whom they had trusted to guard their backs.
Arthur Miller, of all people, wrote a moment like this in Focus, in which Mr. Newman and Abe fend off a bunch of para-Nazi thugs with baseball bats.
In Bound in Blood, the fifth novel in P.C. Hodgell's Chronicles of the Kencyrath series, Jame and Gran Cyd hold off a good number of assailants like this. A rare moment of teamwork for Jame, who's better known as a loner.
Twin brothers Boindil and Boendal from Dwarves do this all the time. They usually run around back-to back in battle, enabling them to cut through groups of enemies.
In The Dresden Files novel Changes, Sanya and Murphy go back-to-back during the final battle with the Red Court of vampires. And by "the Red Court" we mean all of them.
During the same scene, Harry and Molly do this as well, using impressive spells to stall the Red Court army (Harry a gigantic field of ice, Molly her seizure-inducing light show) while Thomas and the two mentioned above slice up the stunned and baffled vampires.
Thomas and Lara Raith do this against the Ghouls in the Deeps.
Simon R. Green books featuring Hawk & Fisher have the titular Battle Couple frequently fighting like this. This is especially helpful for Hawk as he lost an eye and needs Fisher to watch his blind side.
As revealed in the Warrior Cats book Secrets of the Clans, young warriors are taught this as a tactic.
The In Death series has Eve and Roarke doing this on at least two occasions.
In Assassin of Gor, the climactic fight scene in the Stadium of Blades begins with Tarl facing down six of the enemy single-handed, then as more enemies appear first the enigmatic gladiator Murmillius helps with this trope, then Relius, Ho-Sorl and Ho-Tu join in, forming a pentagon of extreme badassery.
In Catching Fire, Katniss, Peeta and Finnick during the Quarter Quell.
Frequently used in Time Riders, particularly in the second book, which takes a survival horror edge with the enemy being a pack of highly intelligent predatory dinosaurs. Justified, in that while Becks is a robot and doesn't really have blind spots, not having to worry about her back allows her better computation for what's in front of her. Also, in the finale, She covers the backs of the hero Liam and the remaining castaways so they can reach the time portal in more of a 'Save yourself, I'll hold them back' gesture - Becks is not only physically tougher and faster than the children, but she also believes herself expendable.
In Andromeda, Dylan Hunt and Rommy made a career out of this. Harper and Tyr Anazazi did this, too, when fighting the Magog together, but they were overwhelmed.
In Stargate Atlantis, Ford and Ronon end up doing this briefly when they're surrounded in a Wraith hive ship before being overwhelmed. In the alternate future where Sheppard disappeared, Ronon and Todd the Wraith, of all people, did this in their last stand in one of Michael's labs. After they killed all the hybrids, they came within a hairsbreadth of killing each other. At the end, Ronon detonates a C4 planted on the computer core behind them, killing them both and all of the hybrids.
Ronon (sword at Todd's throat): Sorry, force of habit.
Todd (knife pointing towards Ronon's heart): Indeed.
Done by Sarah and Bryce in Chuck. It's this sort of display of solidarity between them that makes Chuck uncomfortable.
Sarah and Chuck do the same thing (while handcuffed together) in Season 3.
Chuck and Casey literally fight back to back when they're tied together in two chairs in the first season. Though that was more Casey fighting and occasionally flinging Chuck around to kick enemies in the face while Chuck screamed a lot.
Batman and Robin. The Adam West and Burt Ward version especially loved this trope. (To the point that this was refered to then used, though slightly subverted, in Blankman.)
Used and subverted in the revived Doctor Who episode "The Doctor Dances".'The Ninth Doctor and Captain Jack Harkness find themselves back-to-back in order to defend themselves from an onslaught of faux-zombies... armed with a banana, a sonic screwdriver and a sonic blaster running low on power. While both characters are decidedly badasses in their own right, the subversion was a welcome one - particularly for such a dark episode so far.
The Eleventh Doctor and River do this during "Day of the Moon"'s big fight scene. Of course, River's not sure how he and the sonic screwdriver are of much help while she's blasting aliens.
Power Rangers does this a lot, usually during the traditional team-ups, in which the current Rangers will generally pair off with the Ranger of corresponding color from the previous season in the big final battle.
Angel and Spike do this during Angel's 5th season episode "The Girl In Question". Parodied, as they end up hitting each other.
Burn Notice has Michael and Fiona or Michael and Sam pulling these almost every other episode. Of course, we are talking about an ex-CIA operative, a former terrorist for the IRA, and an ex-Navy SEAL. It's kind of their hat.
Farscape: The Peacekeeper Wars has an amusing variation on this near the beginning. When John and Aeryn are revived in mid-embrace to find themselves in the middle of a room full of strangers, they don't even bother turning around. They just aim their weapons past each other in a front-to-front badasses moment.
Used by Turk and The Todd in a dream sequence on Scrubs.
Liaoyuan Huo and Lu Bu teamed up to kill Dong Zhuo, who was one step ahead and ambushed them with a izable force, plus two skilled assassins. By the time reinforcements arrive, all the ambushers are dead.
Liaoyun Huo and Sun Ce faced off an entire army back to back. Or rather, Liaoyuan Huo and 'Sun Ce', a.k.a. Ling Cao.
Gan Ning and Huang Zhong against about 5 very pissed off elite generals of Sun clan. And Huang Zhong gets them both out of it with some helps from the sideline and the spirit of camaraderie.
The Breaker: Chun Woo and Shiho in the Koa Tower fight.
DJ Format, "3 Feet Deep": "Back to back in a pose similar to Jean-Claude" (Van Damme).
The band, Underdog sings about this in their song, "Back to Back," both metaphorically and literally.
As does Hammerfall in their song "Back to Back."
The quote at the top of the page comes from a Pretty Maids song, appropriately titled "Back to Back".
In Magic: The Gathering there are is a pair of samurai brothers (click in the "Other variations" to see the other card) in the Kamigawa block, who when played at the same time gain bonuses to attack and defence because they are fighting back to back.
In Warmachine, the Great Bears of Gallowswood from Khador are a trio of survivors/good buddies who basically stick back to back to back and slam into the enemy like a wrecking ball of death.
In the Polish RPG Neuroshima, there is a system of friendship, with on of the highest levels being named "Let's stand back to back and pull out the guns".
Done by Trish and Tron Bonne in the second episodic trailer for Marvel vs. Capcom 3, then again by Zero and Chun-Li in the fourth trailer.
Mega Man games in general tend to employ this if there happen to be two or more qualified badasses in the main cast. Mega Man X uses it a lot, since the games' main heroes were deliberately designed and programmed to be badass.
Angela and Arata do this at one point Princess Waltz. Not only does this wind up looking totally awesome, but given the genre, this is also where Angela's respect for him jumps up considerably and provides the bridge to a possible relationship later. In short, this woman apparently finds being Badass a turn on.
Your party does this during the cutscene immediately preceding the first Rachni attack on Noveria in Mass Effect 1. Another really subtle example is only obvious if you draw your weapons manually without any enemies in sight: the moment you do so, Shepard's squadmates on both flanks drawn their own weapons and simultaneously turn around to face in the directions Shepard is not watching.
In Mass Effect 3, two entire species, the Turians and the Krogan, go Back To Back Badasses to push the Reapers back from the Turian homeworld in a spectacular showdown that becomes known as the Miracle at Palaven.
In Dragon Age II, the Rogue class skill "Back-to-Back" allows the user to briefly perform Back To Back Badasses with any ally.
Bayonetta and Jeanne do this a total of three times throughout the game. Once during the introduction, as the plummet toward the ground surrounded by angels. Then again the Epilogue chapter and in the ending cutscene.
Roxas and Riku also do it in the epilogue of 358/2 Days. Most of the army of heartless flees instantly
The Kingdom Hearts series actually uses this trope frequently: Sora & Hercules, Sora & Auron, Sora & Axel, Sora & Riku, and there's also an example of a 3-way back to back with Sora, Donald, and Goofy in the cutscene before the 1000 heartless battle.
Mickey has a literal back to back badasses attack dubbed Holy Burst, where he and another Keyblade user link arms back to back and spin firing Light blasts in all directions. He's done it with Riku, Aqua, and Ven, but notably not Sora (who he rarely if ever helps out directly).
Solid Snake and Raiden do this towards the ending of Metal Gear Solid 2, with Snake initiating it and Raiden obliging. Carnage ensues.
In Metal Gear Solid 4, Raiden comes to help Snake in a seemingly impossible fight. And even though it's entirely a single player game, it turns to split screen. And it really feels like you are fighting together.
Also Meryl and Johnny Sasaki (Yes, the guy who used to spend the games with crippling diarrhea) doing this in the finale, in a sequence that looks almost like it was taken straight out of Mr. And Mrs. Smith.
Which isn't that far off base considering that they proposed to each other in the very same scene.
Vergil and Dante pull this one in Devil May Cry 3, against a huge demon towards the end of the game. Dante tosses Vergil one of his guns, and they do a synchronized, flashy gun-spinning sequence ending with the two back to back, with Vergil holding his pistol sideways over Dante's forming a 90 degree angle and tossing off a one-liner as they blast the living hell out of the demon they're facing.
Earlier in the game, Lady and Dante get a Back To Back Badasses moment as well.
Meta Knight and Marth do this at one point in the Subspace Emissary part of Super Smash Bros. Brawl when surrounded by a horde of enemy Primids.
Similarly, Zero Suit Samus and Pikachu are assailed by two dark copies of Samus' Power Suit on a recently isolated platform. The scene ends with the dark suits facing the two, who have gotten into this kind of pose.
Similar to the Dune example under Literature, in the few cutscenes that show multiple party members, the characters in Drakengard will fight in groups of two or three this way.
In Halo 3, although the gameplay quickly degenerates into "run around and kill all the Flood you can see", the Master Chief and the Arbiter, quite possibly the two most badass characters in the universe, strike this pose after killing the Prophet of Truth and the Gravemind'ssudden but inevitable betrayal.
The Final Battle in Fate/hollow ataraxia ends in a massive battle between the protagonists and an unlimited number of shadow-wolf-things, swarming and blanketing the entire city. Assassin and Kuzuki Souichirou fights back-to-back at ground-level, surrounded by hundreds, and eventually thousands, of shadows.
Uncharted: Drake's Fortune has this with Drake and Eddie doing this when surrounded by The Descendents. "Drake, I just want you to know...I hate your guts." "Likewise pal, NOW LET'S DO THIS!" Cue action sequence.
Odd example here: In Fire Emblem: Blazing Blade, there are two characters, Dart (a Pirate/Berserker) and Geitz (a Warrior), who're like Blood Brothers, and also two of the strongest characters in the game. On the overworld map, if you place them adjacent to one another, their sprites are back-to-back. Whether or not this was intentional or pure coincidence isn't clear.
In the finale cutscene of Part 2 of Fire Emblem: Radiant Dawn, Ike and Gatrie, while on a rescue mission rescuing Lucia, strike this pose for a split second before the two resume their ass kicking of the Crimean Rebels.
One of the special missions in Pokémon Mystery Dungeon: Explorers of Sky involves Grovyle and Dusknoir working together to survive in the bleak future while the main characters are fixing time. The two can be seen in this pose in the mission's official artwork.
In Pokémon Diamond and Pearl, your childhood friend and longtime rival, <INSERT NAME HERE>, challenges you several times throughout the story, usually beats each Gym before you can reach it, and basically gives the impression of staying one step ahead of you no matter how many times you beat him. But at the end of the main story, as you face down Galactic Commander Cyrus and his two main lieutenants, he decides it's time to put rivalry aside, and jumps in for a back-to-back 2v2 battle against Mars and Saturn.
It's worth mentioning that in every other case where you play 2v2, your 'partner' has but a single Pokémon to contribute - and if he goes down, you're left fighting 1v2. The Rival, on the other hand, has a full set of 6 high-level 'mons, including a Munchlax - which only appears in honey trees with an encounter-rate of 0.2%! Damn you, Rival!
Two Words:Mario. Brothers. I mean, come on! The M&L games have them almost literally back-to-back. Of course, sometimes they end up just beating EACH OTHER up for unexplained reasons. But, they gang up on you in SSB, don't they?
Although it never actually happens, much of the promotional artwork for the fifth (and so far final) Legacy of Kain game, Defiance, featured Kain and Raziel doing this. Also invoked, but again not actually shown, in Blood Omen 2 when Kain was remembering the backstory of the boss he had just defeated, his former Dragon, Magnus.
In the opening video to Tales of the Abyss, Luke and Natalia do this while surrounded by a squad of Kimlascan soldiers.
This seems to be the preferred state of existence for Flynn and Yuri in Tales of Vesperia. They do it on the game cover, promotional artwork, the cover of First Strike, standing up in-game, sitting down in-game...
The realtime intro of Dynasty Warriors 3 has two characters (I believe Guan Yu and Zhang Fei) doing this while encircled by enemies before splitting off to fight said enemies. Bonus points for being customizable and allowing any two members of the cast to do so.
The Clank Zapper in the second game is a straighter example; Clank remains an Awesome Backpack, but now electrifies any enemies within range, and also has laser eyes aiming at anyone directly behind Rachet.
In the Game Of Thrones RPG, the two main characters do this when a squad of guards enter the throne room during the final fight.
Most versions of Bad Dudes (including the arcade and NES versions, but not the Japanese Famicom version) have the dudes posing like this on the title screen.
Ronin Galaxy: Page 19, when they come to Taylor's rescue. Despite the fact that she almost killed them on accident.
Also occurs briefly in "Pirate Radio," with Danny and Dash battling a group of... well, pirate ghosts.
Jack and the Scotsman pull this one every time the Scotsman shows up on Samurai Jack.
Don't forget the Spartan King.
This happens in the two-part season 2 finale of Transformers Animated with Optimus Prime and Megatron fighting back to back against Starscream's clone army... until Megatron grabs Prime in his hands and starts using him as a shield.
Transformers Prime. Optimus and Ratchet against a whole army of zombie decepticons. 3 guesses who won.
And pulled for a third time, with Optimus and Megatron while fighting inside of Unicron.
Subverted in My Little Pony: Friendship is Magic, as the protagonists are accosted by a small army of changelings. Twilight Sparkle and Applejack back up against each other and rear up, and it looks like they're about to kick some serious faerie butt until "Twilight" changes back into a changeling.
Applejack: Okay, this is just gettin' weird!
Avatar The Last Airbender - Main characters Zuko and Aang are about to face the potential threat of the firebending masters, and they don't know whether or not they will assist or attack them. They invoke this trope to be on the safe side. Turns out to be a good idea, since the masters are two great big dragons, although they do not attack.
Also inverted in an earlier episode, when Zuko and Jet are battling each other, they end up with their backs to each other, each turning and trying to get around the other's defense.
Played straight with Zuko and Katara in the gaang's training session right before the Grand Finale.
Subverted in The Venture Bros. in the episode "Tag Sale — You're It!" Dr. Orpheus tries to invoke this when the villains run amok at Dr. Venture's yard sale. Venture, however, wants no part of the chaos and goes inside his house.
Link and Zelda in the first episode of the obscure The Legend of Zelda cartoon. Although such a thing seemed implausible since they were tied together, which would normally weaken their fighting.
Faragonda and Griffin in Winx Club get such a moment when the Trix invade Alfea.
In the 2007 TMNT movie; Leonardo and Raphael end up doing this whilst fighting the Stone Generals in the climax.
In the climax of Justice League Unlimited, Lex Luthor and Batman wound up doing this together while fighting Darkseid's army. Earlier in the episode it was Batman's suggestion Lex be allowed to fight alongside them to defend against Darkseid.
In a recent episode of Wakfu, Evangelyne and Remington (who were, up until that point, fighting each other - mostly to a stalemate) pulled this off when they were surrounded by zombies. It was almost immediately subverted as they were quickly and easily overwhelmed and captured.
Generator Rex: Rex himself actually says this word-for-word when he and his enemy Van Kleis are fighting sand monsters in the middle of a nanite-destroying zone.
Van Kleis: Before were totally devoid of nanites we need to end this now, together! Rex: Back to back! Van Kleis: What!? —>Rex: Haven't you ever read a comic book? Back to back!
In the Thunder Cats 2011 episode "Old Friends" Captains Grune and Panthro do this while quite unfazed by the long odds against them on a battlefield full of their Lizard enemies
Grune: If y'wanna make it ta general, you'll have to do better than that. Panthro: *laughs* I'm eleven lizards ahead a' you Captain! Grune: *swings his mace, hitting two Lizards, laughs* Make that nine!