Connecting 2 speakers to 1 output on a stereo HIFI system?

HiFi speakers are only connected parallel, that is because the internal impedance is complex, especially near the self resonate frequency. Series connection would allow one speaker to choke off the current to the other speaker if they have different self-resonate frequencies so overall performance of the combination is more impaired than the individual contributions. Modern transistor HiFi amps have a very high damping ratio, which means their characteristic output coupling impedance is much less than the nominal speaker impedance. Old fashioned tube amps with transformer coupling required impedance matching. So no, the amp is not likely to be bothered by the lower load impedance at all. that being said, the amp does have limits on its ability to source current, so you most likely won't get the same rated power from the amp. so adding speakers does not increase the output power, and could lower it somewhat. you use additional speakers in order to distribute the sound more effectively, so that for a given listening position it might sound better or louder, or not.