speakers on/off

If you are wrong about the common earth - and a lot of amps do not use one - then you will be in big trouble if you common speaker cables.
Using a single pole switch on a common return is not a good idea. The output of one channel will drive through the speakers back into the output of the other channel. If the amplifiers are direct drive outputs, there could be a dc offset between the channels which could cause damaging currents to flow where they shouldn't. Even if the amps aren't direct drive, the speakers could easily still produce sound, and quite a lot of it, with the common disconnected - as one channel sinks the output voltage produced by the other. "Running backwards" like this could cause electrolytic capacitors to be reverse biased and blow up.
But I guess that you have been told all that.

Connecting the outputs through a load back to the other channel will not cause harm (common supply amps). It is the basis of bridge amplifers. Of course, in stereo mode, the common signals produced would cancel out, leaving a kind of surround effect. Connecting speaker loads between the + terminals of direct connect and capacitor coupled amps was used to produce the fake 4 channel sound back in 70s. Of course, this is not what the OP wants.
With capacitor coupled outputs, the amp output is biased at 1/2 the supply voltage. Connecting the outputs together through a load, no harm will come to the capacitors.
Of course, this implies the amp is functioning properly with proper load impedance. John

please dont do this. use a double pole swtch to break both speakers.
your method will not shut the speaker completely off and the greater the stereo content (L-R) the louder the funky sounding audio will be. or if you adjust the L/R balance, the greater you turn the balance the more sound comes out... to 2 speaker wires out of phase.

Thanks for that. You are, of course, right! My only excuses are that it was gone 2 in the morning - and another system was on my mind at the time.
In the OPs case, the amplifiers are probably identical or near identical and are driven from similar, if not common rail(s).
As you say, the result would not be what the OP wanted. So, at least I got that right!

Perhaps someone can comment on the question of switching off the speakers. In the old days, it was sometimes considered bad practice to disconnect (switch off) speakers. The amplifiers could be in danger with no load attached. I have no idea if this would be true in your case. --Phil

assuming solid state electronics it's a non issue. tube equipment or something with a transformer output needs to have a load.
many amps these days have the kind of switch we are discussing built in: where you can have "A" or "B" or both or neither.

On Wed, 25 Jan 2006 09:27:12 -0500, Phil Munro Gave us:
Pulling the loads off of solid state amplifiers is a bad thing... yes.
Better to attenuate the drive of the amp than leave it blairing and pull the loads off the outputs.

what is bad about it?
how can an amplifier 'blair' with no speakers attached?

On Thu, 26 Jan 2006 12:34:46 -0500, "TimPerry" Gave us:
The turn on spikes can damage the transducers (read "speakers") for one thing. On some amplifiers, it can damage the final stage of the amplifier.