I bought one last week and have it pre-amping my Fender 1200. It is awesome for tone shaping. My bass mentor/instructor calls it "god in a box" I have to agree.
My 1x15" R100 doesn't. Just a variable line-out designed to go to recording equipment. I don't see why it couldn't be used (at a low level) as a reference signal to another combo amp.
Wouldn't it be a lot cheaper (and easier) to use the variable line out on your rumble 100 to provide a signal for another one? I'm thinking another 100 is probably cheaper than getting a real good 4x10" cabinet.