Sorry for the poor image quality, but attached is a sketch of the sort of thing I had in mind. Two ganged 3-way Make-Before-Break switches, wired as shown. I've had a couple of beers (in China right now, where it is evening), but I think it does the job of ensuring neither amplifier ever operates with no load, and that the two outputs from the A and B amps can never be connected together as the switching occurs. The indicated positions of the switches are for Amp. A playing through the speaker. The middle position is neither A nor B playing through the speaker. R would be 8 ohms or so. Of course another 2 gangs are needed for the other channel of a stereo amplifier.
The schematic assumes it is allowable to have a common ground for the outputs from amplifiers A and B. If this is not allowable (e.g. circlotron OTL amplifiers) then I suppose it would need a similar switching arrangement for the other end of the speaker too.
I could imagine there are more efficient ways to achieve the desired goals, but I think this would achieve it (unless the beer was too much!).
