Two amplifiers feeding the same pair of speakers?

The simplest, and safest way is to use one power amp, and use your second source as an input to your main preamp. I'm not sure is this gets you what you're after though since you didn't provide details. You may want to describe your situation in detail.

Whatever you do, do NOT have 2 power amps connected to the same speakers at the same time, unless the amps are isolated from each other by a switch.

You can ONLY use 2 amps if the speakers are bi-wirable and one amp going only to crossover section of each driver(tweeter or woofer). If you try to power a loudspeaker with 2 amps one will try to over power the other and one will loose and blow up. I know because that's exactly what I did when I was 16 years old, and fried my father's receiver... yeh he was not happy RIP.

Niles makes a nice little switch box for this reason.

Thanks Elevick; I also ran across this unit after doing more research. Do you actually have the DPS-1 switch box? If so, assume you are happy with it? The only drawback I see so far is that it only accepts up to 14 gage speaker cable.