Whenever a video game has a boss fight against multiple characters, the game can either have each boss with their own life meter, or it can have all the characters sharing a single life meter, with every enemy, regardless of how much they've been personally damaged, inexplicably keeling over once it's been emptied. In some games, the game will balance out the fact that the player can control more than one character by having them share a life bar.
An alternative to this is when multiple enemies technically have their own life meter, but for the sake of convenience they've all been combined into one giant life meter, and emptying it involves defeating any individual enemy (or it might involve one giant enemy with multiple destructible parts that have to be taken out.)
Often overlaps with Dual Boss and Wolf Pack Boss. Sometimes a subtrope of Synchronization.
Examples of characters all getting their health from the same life bar:
The World Ends with You has Neku and his partner sharing one life meter that stretches across both screens of the DS. It even empties at both ends depending on who's getting hurt, with one character displaying their injured idle animation if his or her half of the life bar is completely empty.
Any Dual Boss fights in the game (like Kariya and Uzuki, Kitaniji and Shiki, or Bonus Boss Hanekoma's simultaneous dual forms) have this rule applying for the bosses as well
Forever Kingdom features three playable characters who are cursed with the Soul Bind, which causes them to share the same life force, giving them all one life meter. Unfortunately, this feature was highly criticized.
Mega Man ZX series: Whenever the two are fought together, Prometheus and Pandora share a life bar. Whereas each of them alone has 2 layers of life bars, them combined has just 3. In the first game they just attack alternately, while in the second they get Combination Attacks.
The Superman Returns video game has a health bar shared by the entire city, which goes down whenever enemies hurt civilians or cause property damage.
Both Joe and Silvia share life and VFX meters in Viewtiful Joe 2.
In the Drakan series, the Player Character Rynn and her Bound dragon Arokh share a single life meter, though this seems to only work one way (if Arokh dies, so does Rynn, but if Rynn dies, Arokh seems to survive). This is justified in-story by the nature of their Bond, which magically links the lives of the dragon and his rider, and Arokh has already survived the death of his previous rider in the backstory.
Crash Bandicoot 2: The Komodo Bros share a life bar. Moe is invulnerable, but Joe isn't - and to damage them you have to smack Joe, which will spin towards Moe, hurting them both.
Some of World of Warcraft's bosses have this, typically with the added mechanic that only one of them is either more vulnerable than the rest or the only one vulnerable. Examples include:
The Blood Prince Council in Wrath of the Lich King's Icecrown Citadel.
Mogu'shan Vaults in Mists of Pandaria has two examples of this in its first and last bosses:
The Stone Guard at the beginning will occasionally become invulnerable, requiring players to change targets.
The two main bosses of the Will of the Emperor encounter are both vulnerable to damage throughout the fight. But the encounter throws so much cannon fodder at you it's only possible to really focus on them at a few particular points.
The Trial of the Yuangol in the Mists dungeon Temple of the Jade Serpent. Both start out vulnerable, but as they take more damage they eventually develop into a buff that makes them temporarily invulnerable, requiring the party to alternate targets.
The three main characters of Gobliiins all share a life meter, though this was removed for future games in the series.
Scramble Mode in Street Fighter X Tekken is 2-on-2 with all 4 fighters active at once. Each side shares one life bar between both teammates.
Drakengard: Caim and his dragon share a life bar both when he's riding and on the ground, though Caim has the opportunity to replenish it (by killing enemies such as archers, who can take out them very quickly when airborne).
Warcraft III has the Spirit Link ability, which spreads damage across multiple units.
Basic Dungeons & Dragons. Sabreclaw monsters are created in "wings" of 2d10 members. Each sabreclaw contributes 25 Hit Points to a pool shared by the entire wing. For example, a wing of 10 sabreclaws would have a total of 250 Hit Points. Any damage inflicted on a sabreclaw is divided up among all of the members of its wing. Once an amount of damage equal to the wing's pool is inflicted on it, all of the sabreclaws in the wing die.
Life Points in Yu-Gi-Oh! work sort of like this. Monsters battling each other remove Life Points from each player, and when a player's Life Points reach zero they lose, along with all the monsters they were controlling.
Examples of characters combining their health bars into one.
The first two games in the Serious Sam series will occasionally lock Sam in an arena and hurl massive amounts of enemies that have to be killed, using a boss life meter to gauge the player's progress.
Abobo's Big Adventure features this when Kirby absorbs Krang and commandeers his giant robot suit. The player then has to attack and destroy the robot suit's head and right arm, along with beating Kirby himself, all of which share a life meter.
Mega Man 2 has the Boobeam Trap and Picopico-kun in Wily's Castle, the former being a series of turrets on the walls that all have to be destroyed, the latter featuring wall panels that combine into flying robots.
The first Mega Man game has CWU-01P, a series of robots encased in bubbles.
Mega Man X has Rangda Bangda (which also reappears in X5,) a giant robotic face that's defeated by destroying both it's eyes and its floating robotic "nose".
Mega Man 4 has the Cockroach Twins, two robots fought one after the other.
Muramasa: The Demon Blade is...complicated about this. Bosses have two health bars, one large one and one small one (that keeps feeding the large bar whenever it's depleted while slowly depleting itself in the process.) Whenever multiple bosses are fought at once, they have separate large bars, but they all get health from the same small bar (although the small bar is then split up into separate bars, one for each boss, denoting their own portions of the health meter.)
The second boss of the Game Boy version of Battletoads has a first form with one life bar, but requiring the player to destroy multiple turrets on it before moving on to the second form.
The Supernaut from Rework the Dead: Evil is a partial example, requiring the player to reduce it's health bar by shooting off it's scales one by one. However, the player doesn't need to remove them all to defeat it.
In Mega Man ZX Advent, Argoyle and Ugoyle, when fought, initially appear to have single health bar. However, when you activate Model H and use it to Enemy Scan them, you'll see that each of them has their own health bar.
As of the Soulstorm expansion, the total health of squads in Dawn of War is visible (the earlier games only showed it when the squad was down to a single unit), though individual unit's lifebars are still visible.
Examples of both types at once:
The Last Remnant has this for the squads you build to fight enemies. An especially lucky enemy roll could leave five characters badly injured but alive... ,sharing all of one HP. Conversely, some enemy attacks can KO individuals without doing much damage, couple that with healing spells not automatically reviving fallen units and you can end up with a single wakeful character while the other four are KO'd
Five hats means that five tropers think it is ready to publish.
You are saying that you think this draft is ready to be published. That means the description is not ambiguous,
it doesn't duplicate an existing trope, there are at least three examples, and the title makes sense.
Is that what you meant to do?
You are saying this draft has a ready-to-publish hat it does not deserve and you are taking it back.