Assuming he has wired them properly, if he uses a Yamaha receiver-amp with one set of speakers and another identical receiver-amp with the other set, he will improve the systems sonic qualiies compared to just using one amp for all speakers. He will have created more headroom with more power on reserve. That typically means improved dynamic range and less distortion especially in transients.
Two amps are sharing the burden of all the speaker load instead of one.

John