Hi Danr,
Ravi and mwc are correct. Don't even try it without getting a speaker selector--in this case, an amp selector--because if you were running one amp with the other off and no switching, the impedance load that the speaker presents would be altered by the extra set of speaker cables and the dormant amp circuitry. In other words, your second amp would be trying to drive the speakers and the circuitry of the receiver.
You would need a selector switch that accepts the sets of speaker cables from your amp and the receiver, with one set of stereo output cables going to your speakers. The switch would select either your receiver's output OR the amplifier's output, disconnecting the source amp not being used.
Regards,
