Is there a mod that would allow one computer to host the nether and the end and the another to host the normal world? Or one server host survival and another host creative, but making them intertwined so that players could simply "teleport" between them.
You could run multiple instances of a minecaft server on the same box if you have enough resources. As for teleporting to another server..I think I say something like that here before. I'm not sure.