A common quirk of a creature with multiple bodies, or 2/more creatures/person who are "linked" in a deep manner, or a Hive Mind group, is that all of them have to be defeated at nearly (if not exactly) the same time, otherwise the killed individual will regenerate/revive itself.
There are many explanation for this, including but not limited to: extreme regeneration ability that's apparently shared by two people, ability to duplicate themselves or breed a huge number of them in a short time (for Hive Mind cases), or the individuals being able to heal/resurrect their own dying comrade. Killing all of the entities at once would thus prevent any individuals to come back.
- In Neon Genesis Evangelion, Israfel was actually two entities which had to be destroyed at exactly the same time or else they would keep coming back. Similarly, Iruel was a nanovirus that kept growing and adapting until they managed to trick it into an evolutionary path that led to self-destruction.
- The Twin Card in Cardcaptor Sakura needed to have both halves of it defeated at the same time, and Syaoran and Sakura found that attacking them separately was ineffective. Luckily Meiling had trained with Syaoran in martial arts and they were able to carry out a coordinated attack to defeat it.
- Demon Slayer: Kimetsu no Yaiba has the Upper Rank 6 demons Daki and Gyutaro, who due to the nature of how they became demons count as Two Beings, One Body and, while they can separate to fight individually, they both need to be decapitated at the same time (or at bare minimum the other needs to be beheaded before the other can reattach their head) before they can be fatally injured.
- In The Lucifer and Biscuit Hammer, one of the golems the beast knights had to fight as part of the evil mage's challenge showed up as a pair, and then some. One was a straightforward strong centaur like golem, the other a swarm of almost cute mook like minions. They beat them, but the next day the swarm showed up again, but slightly different, and stronger. This continued for several days, the Beast Knights knowing that they had to figure out the trick to making them stay down or their slow improvement would let them overwhelm the Knights. They deduced that they needed to kill all the golem's bodies for it to stop respawning, and that at least one body was hiding while the rest showed up to fight each day. Eventually one of the beast knights found it, killing it easily because since it hadn't been destroyed unlike the rest of the bodies, it never came back strong.
- Soul Eater: Troubled Souls: This is the main reason why the Brother and Sister of the Gemini were such a difficult Kishin Egg to locate and kill. They are actually two entities which had to be destroyed in their combined form or else they would keep coming back. Maka and Caius found attacking them separately is a fruitless endeavor because the "killed" half would simply find the other, recombine, and then split again into two beings good as new. To make matters worse, the separate halves are big on working together to survive.
- Robot Wars entrant Gemini is the Trope Namer. If 50% of any competitor's weight was destroyed/rendered immobile, the competitor was counted out; for Gemini, which was a clusterbot that split into two smaller robots after entering arena, this required either completely destroying/immobilizing one of the two robots or destroying/immobilizing 50% of each robot. This rule was removed later.
- Mitosis: The titular Epic has the power of at-will duplication, with no "prime" copy. So as long as even one of him exists, he can have a hundred bodies again within a minute.
- Dungeons & Dragons 1st Edition supplement "Fiend Folio": The Cifal (Colonial Insect-Formed Artificial Life) was a monster made up of a huge number of insects bound together into a humanoid form. It had 10 hit dice altogether, two of them being the binding energy that kept the insects together. If you did two hit dice of damage to it, the insects broke apart and fled. That is, if you did 20% of its total damage, it was defeated. Course, then you've pissed off a whole hive of moderately intelligent bugs, who, being demonic, are probably going to want revenge...
- New Destroyman in No More Heroes 2: Desperate Struggle is a partial case. The Destroyman who can fly and attacks mainly through projectiles can revive the other Destroyman. The other one, who fights mainly through hand-to-hand combat, has no such abilities. However, the one who can fly spends most of the battle out of reach, making it hard to actually finish him off first before the close-range one pummels you.
- Azure Striker Gunvolt features a Sumeragi Adept named Elise you must fight who splits into two bodies for her Boss Battle. If you fail to destroy both bodies at the same time, the surviving one will revive the dead one to full health. This can get tricky despite Gunvolt's ability to tag multiple targets for his flash field since the frenetic nature of the fight means you can double tag one body and deal it more damage than the other, throwing off your timing. Elise returns for a second fight at the end of the game, along with all the other adepts, revealing she actually possesses THREE bodies (the reason she survived to revive everyone else). Unfortunately for Gunvolt, this third body is immune to anything he can throw at her, but fortunately, hes not the only enemy of Sumeragi in the area.
- Subverted ironically with Mega Man 3's Gemini Man, who does split into two bodies but allows you to destroy each body in sequence instead.
- Mega Man & Bass has an enemy called Plasma±, resembling two green orbs with an electric current between them. Destroying one will cause the remaining one to respawn it if not quickly destroyed.
- In some Mega Man Battle Network Bonus Dungeon, there are some preset encounters containing (usually) three identical enemies with infinite HP regeneration and ability to instantly revive themselves if you take them out one by one. To win these fights, you have to kill all enemies with one attack at the same time.
- In Tsukihime, Nrvnqsr Chaos can only be killed if all 666 creatures that make up his body are killed at once. If they are killed individually, he can instantly regenerate them. This fact makes him nigh-impossible to kill, even for top-tier fighters like Arcueid. Unfortunately for Nrvnqsr, protagonist Tohno Shiki has the ultimate counter to this in the form of the Mystic Eyes of Death Perception — when he kills something, it stays dead, no matter what. So he's able to kill pieces of Nrvnqsr without them regenerating, which freaks the monster out.
- Lufia II: Rise of the Sinistrals had a dungeon boss that split into two, one form being fire-based and the other ice-based, which had to be defeated simultaneously. When the party first fights it, they fight each body one at a time; it regenerates and laughs in their faces. So they split the party in half and two characters fight each body.
- Final Fantasy V has the Gargoyles that guard the tablets used in the Sealed Temple. Killing one will just cause the other to revive it; to defeat them, you must kill them simultaneously or prevent one of them from acting (such as by sending it berserk).
- Final Fantasy VIII has the Propagators, four color-coded pairs of creatures hiding on the Ragnarok. To defeat each pair, you have to kill one after the other without getting into a fight with another Propagator of a different color in-between. If you do, the defeated creature is revived by its partner.
- The Legend of Zelda: Twilight Princess: Shadow Beasts always attack in groups of 3 or more, and when only one is left standing it will let out a howl that revives the others. To defeat them you must use area attacks to take down the last two at once.
- Persona 3 has Chariot and Justice, a Dual Boss fight where both targets have to be killed in the same turn, or else the survivor will use the Samarecarm spell to fully heal the other. The two bosses occasionally fuse together, and one may think to kill them in this combined form...however, doing so causes only one of them to be destroyed and the other to revive them, so they must be killed while separated.
- Sleepless Domain: Cassidy's power as the magical girl Flash Cut is to split herself into two halves. These clones are significantly less durable than Cassidy herself, but if either of them is destroyed, Cassidy herself reforms unharmed at the intact clone's location, and can continue to split herself for as long as she still has enough magic to spare. If both of her clones were destroyed at the same time, however, there would logically be no intact clones left to become her main body, and Cassidy wouldn't be able to reform. Which is exactly what happens when Goops gets a monster to slash at both clones. Cassidy uses her Dying Moment of Awesome to stab Goops's eye in retaliation before she disappears.
- This is the schtick of a fae archer the party of Tales from My D&D Campaign runs across. He can separate his shadow from himself, which in addition to doubling his firepower means that whenever either his shadow or his real self is threatened, it can momentarily teleport to the other's location and then back again to dodge attacks. The only way to harm him is to threaten both his bodies at the same time.