Hooking up 2 amps to one set of speakers

Don't do it. The first time you make a mistake and leave (or turn) them both on you will fry one set of amps, or a receiver.

Lots of concerns. Outputs of two amps (at least solid state amps) should never be connected together, and what you are proposing to do does exactly that.

It's hard to predict what will happen without specific design details on each amp, and what will happen will depend on whether the one that is not being used is on or off, but it is highly conceivable that damage to either or both amps can result.

Even if damage does not result, if the unused amp is on, it would have a very low output impedance, likely a small fraction of an ohm if it is solid state, which would cause excessive loading of the amp that is in use. If the unused amp is off, its output impedance is probably undefinable, with unpredictable loading effects on the other amp.

Don't do it!

Regards,
-- Al

Have you considered a speaker A/B switch. Connect the 2 amps to the speaker connection and the speaker to the amp connection. As far as I know there is no real direction in a switch. If I'm wrong I'm sure someone will chime in. I would also suggest looking for one that just does A or B and not both so there are no accidents.

Yes, a switch is non-directional, at least for those of us who believe that things like fuses are non-directional :)

"One that just does A or B" refers to what is called a "break before make" switch, which is definitely the kind to get (as opposed to a "make before break" switch, which would momentarily establish a connection to both amps as it was being switched).

If you use a switching arrangement, it would be good practice to be sure to have the unused amp turned off at all times. Tube amps generally cannot be safely operated without a speaker load; most solid state amps I believe can handle no-load operation ok, but there is no point to taking any chances.

Also, keep in mind that a switch can probably be expected to have sonic effects comparable to or greater than the differences between speaker cables, if you are one of those to whom that may be significant.

Regards,
-- Al

I am glad I asked. It seems pretty clear that this option is completely out of the question. Thanks for your responses.