cool. Mmm.... how can you make two CPUs fight? I think I didn't do it, but I might be wrong.
If you don't assign any controllers to either side on the Character Select screen ( press A button or J key without moving any of the controllers to any side), then you are allowed to make 2 CPUs fight each other. Their AI isn't actually programmed yet, so they'll just stare at each other until the timer goes down (you can still pause by holding Esc.)