If you switch both the input and the output of the amp simultaneously, having the unused amp going into a resistor for load purposes and simply switching the inputs, it will work fine. (I know there are some who will disagree..but it works for me!)
The switching has to be sequenced so that neither amp never operates without a load and is never connected together (except via ground, ultra important!)
There are a few devices (such as the Head Trip) that do this. They're not cheap, though. A simpler approach might be to wire the cab for stereo (assuming that the speakers can handle the power of both amps).
The device Sonny Landreth uses is a Palmer E-Frog amp switcher, according to an interview I read, though I know nothing about this particular device. I've seen pricing on similar units, though, and they weren't cheap.