yes... you can use a y-adapter to feed 2 signals into one amp. but you have to make sure you turn off one signal when you turn the other on.
or there are A/V switch boxes that you can use to select what input feeds into the amp. just dont hook up the video cables, and you can use it for audio only. they sell these at radio shack and almost every electronics store.
dont know if these are feasible options, but the options are out there. i wonder how good either will work...