Submitted by @your-favorite-sickie – thanks!
A and B are on the same team, but are captured by the villain and forced to fight each other to the death (the winner gets their release), or else something way worse will happen (a city will get bombed, their other teammates will be killed, etc.). If they refuse to comply in a timely fashion, they will both be killed instantly.
Cue the heart-wrenching “I don’t want to hurt you!” and/or “Just kill me now. At least you’ll be okay.” Also cue the death scene of one succumbing to their wounds on the battlefield, or maybe a hospital scene (they could survive or die).
Bonus points if one character is significantly stronger than the other; no matter how much they hold back, they tragically know who’s gonna win.
Double bonus points if the characters are close beyond being teammates (lovers, siblings, etc.).
Oooh yes!!! and for those who’d like a somewhat happier ending – their teammates burst in just before B gives the killing blow, they rescue them, there’s some arguments based on the fact that B was about to kill A but eventually they work it out, the angst thought! Ooh, or one of them could be drugged/ temporarily brainwashed into believing they have to fight while the other character tries to break them out of the control, they only come to their senses when they’re sat next to A’s bleeding and trembling body.