Two amps into one pair of speakers

Does his speakers have separate binding posts to send signal to different parts of the speaker? For instance, many speakers have four binding posts in the back. A positive and negative for the woofers, and a positive and negative for the midrange and tweeters. If this is the case, then your friend is bi-amping his speakers. Typically one would want to ensure proper level matching through independent volume controls, or at least one should be able to gain match one amp to the other.

if your friend is using two amplifiers to hook up to the same set of binding posts, he shouldn’t as it could surge the amps, his speakers, and potentially his circuit/house. 

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.