Outputs of 2 amps share common return - problem?

I assume when you say return you mean the black speaker post?

If the black post on both amps is connected to chassis ground then it won't hurt anything, but you could create a ground loop and some hum. You can use an ohm meter to check, just make sure the amps are off when you measure.

If it is not, then I wouldn't try it. On some tube amps this post is floating and on some SS amps the output stage is balanced so both speaker terminals are hot.

I know it is a pain but you would be better off moving the speaker cables to the amp you want to use. Or modify your switching scheme to switch all wires.

Make sure the amp not being used is off. You can damage the ouput stage without speakers hooked up, especially the tube amp.

In digital amps (all I have seen) both speaker output terminals are hot wrt ground. So don't try this switch with a digital amp. Otherwise, no problem.

I would not do this nor recommend it. I'm sure that it has been done a million times before, but that doesn't mean that it works well or you won't be the one to experience problems. Sean
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Thank you guys for responding. I think I'll be rewiring the speaker cables so all 4 are switched, especially that I prefer to keep my Mc amp ON 24/7.

Sean...Unless the interconnects are disconnected (or switched) all the amp grounds are tied together by the signal returns, even if you go to the trouble of disconnecting the speaker returns. (Assuming RCA interconnects). If you follow your argument to its logical conclusion, one should disconnect all unused items in the audio system.