Something like a three-pole toggle (using the middle position as OFF).
or you could buy one
http://www.amazon.com/Niles-DPS-1-Black-FG00003-Audio/dp/B00006HOFR
Niles Audio had a switching unit for this. Not the random part others have mentioned, but definitely the two amps to a single speaker setup.
https://www.amazon.com/Niles-Black-FG00003-Amplifier-Selector/dp/B00006HOFR