If you have a large boss level you can try the following
(It abuses the game engine so some people might consider this cheating)
Run to the furthest point from the exit/entrance thingie.
Use one char to attack one of the two bosses (need a damage/miss text)
Let that char run around in circles at that point.
The other char attacks the other boss until it chases that char.
Now pick one of the two chars, run back to the exit, let that char run around in circles there.
Now rush the other other char to the exit.
do a save then load that save.
and voila you should only have one boss to worry about.
