I don't think there are any rules that prevent them from fighting each other. It's probably safer if they agree to terms like "fight until first bloodied" or "fight until zero hp" rather than "fight to the death". As for the actual fight, I imagine it would rather boring and unsatisfying, since the game revolves around synergistic powers across party roles rather than a lone character fighting.
If they're just sparring, I would make them design the map together—this is their characters finding and agreeing on a sparring space. I would also just use an ability check vs ability check, using the modifier associated with their class's attack powers. Actually doing an entire fight is a waste of time for the group; if they want to spar, let them do it on their own time.