The AB switch would be the easiest solution...you just need to set the master volumes on the amps appropriately.
If you are not going to use them together I woul consider going to a switching rig. I have been contemplating this for a long time, and I have most of the components. Basically, the switcher is a series of loops, midi controllable. You put each amp/preamp in a loop, effects in other loops, and output the end of the switcher to your amp and speakers. Here is the Rocktron Patchmate link if you are interested.
Rocktron - PatchMate Loop 8