Well what does your cab run at?8ohm's?I would wager it runs a 8ohm's.Best to have your head set to that.

Usually guitar cabs run at 8ohm's...put two together to have a full stack and then you might want to switch to 4ohm's.

I have no idea about running two heads into one cab at the same time,but I think you can have an amp selector set up so you can kick in whatever amp you want.I think "voodoo labs" make an amp selector.Just be carful with tube heads,since turning them on without a load(connected to speakers)can damage the amp.

hope that helps.