Two amps into one pair of speakers

That is pretty dangerous if someone accidently turns one amp on while the other is also on. It will happen one day.

I have a preamp that outputs to 2 amps which are connected to 2 systems, a 2-channel and a headphone amp. I also make sure only one amp is on at anytime, however, I will not have a blowup if I accidently have both turned on. That has happened a few times.

@yyzsantabarbara 

+1 Absolutely. 

i had a mcintosh speaker that you could connect 2 amps to it! 1 amp powered the woofer and the other powered the mid and highs!

I'm guessing from the OPs description that it's not an issue as long as they are not both sending a signal that it's two amplifiers connected in parallel and not a bi-amp setup.

I have a home theater receiver and Pathos integrated connected to my speakers in parallel, but I have an intermediate toggle switch in the speakers cables so that only one amplifier can send a signal to the speakers at any given time.  The custom switch even has a resistor to provide protection during the switch since it was first used with an all tube amplifier that needed a continuous load.

Not the best practice, the unused amp becomes part of the circuit the working amp is driving. Power is feed back into that amp. Some amps don't like that.