The second one isn't open-ended, though. It seems that way at first, and that would have been awesome (in my opinion), but it's still totally scripted. Enemies don't go to one or the other - they go to one. There may be 2 different exits, but an enemy is programmed to only go to one of them.
So it isn't open-ended. You just have to play through once to figure out which wave goes where. If you know that then it isn't hard and certainly doesn't require co-op.
I 5 skulled each level with each character. I never once played co-op.