This topic has been discussed on the old forum a few times.
Running two amps into a stereo cab (or 2 2x12's) is the easiest way to do it. You would plug your guitar into an a/b/y switch, running cable to both amps.
But, to run both amps into a single mono cab is just a wee bit more difficult. You'd need a switch that can put a load on the silenced amplifier while running signal from the other amplifier to the cab.
I don't know who makes a pedal like that...
It would most likely be a custom job. An expensive one at that.