Quite a few of the boxing software options will let you play with characters on more than one system.
Pretty much all of the larger software options, except Pwnboxer allow networking.

If you want to play with hardware boxing, that's an option too.
But I don't really know all that much about this... beyond going with cordless keyboards/mice with a single keyboard/mouse set for the same frequency on each system.
I know there are things like KVM switches, etc... but have never gone that route.