To prevent it from being being just a one player run, both players have to be alive during all bosses and if one dies, and their ghost floats off screen, there's a 5 second window to either kill the boss or revive that person by hitting the plus, or menu, or start button on their controller (which will take an HP if the other player has at least 2, if they don't it won't work). If they are offscreen for more than 5 seconds the boss, and therefore the entire run, is invalid and you'll have to restart the boss when you think that 5 seconds have passed in order to keep the run going. Hopefully that clears it up. This was something my brother and I didn't get when we first did Co-op runs
Another thing to point out is that you can be off-screen for more than this if Player 2 is a "sticky ghost", meaning it's an infinite parry source via the dead player's ghost, but you have to prove it exists in order for the run to be valid.
