I can't offer specifics in terms of brand names, but you would need a multi-contact relay that was switch activated. Using such an approach will probably increase cross-talk between your left and right channels. The only way that i know to get around this would be to use two relays, one for the left channel and one for the right channel. Both could be run from the same switch for sake of convenience.
Things to look for would be coined silver contacts and high current ratings. I would also look for a relay that used as low of a switching voltage as possible. That is, you'll have to have voltage feeding the relays, which will place your audio signal in very close proximity to the field of the coil used to trigger the switching of the relay.
If you were to prefer just to use a switch, you would need something that was a four pole double throw unit. This switch should specify that it was a "break before make" type of device", otherwise you'll blow the tar out of your amps. Once again, high current potential with silver contacts would be preferred.
In terms of wiring, the speakers would go to the center taps with Amp A on one side of the switch and Amp B on the other side of the switch. Obviously, you would want to pay close attention to maintaining proper polarity in all of the connections.
Bare in mind that using a switch or relay here will introduce impedance changes into the system. Other than that, be VERY careful should you choose to go with some type of commercial speaker switchbox. Many of these make / break only the "hot" lead, leaving the grounds common at all times. This may be fine if all of the components are tied into the same outlet / circuit, but if you have two different systems on different outlets / circuits, you have the potential for a shock hazard and ground loop with the associated increase in hum and noise.
You might want to try posting this over in the "Tweaks" Section over at AA. Very helpful bunch over there with a vast amount of resources and knowledge. Sean
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