Reddit user aerialbyte reports on what happens if you try to use two HomePods in stereo mode. The feature is “coming later this year,” but using two of the speakers doesn’t break quite the way I expected:
I just received the two HomePod’s I ordered! As soon as I added the second one to the Home app, it asked me if I wanted to put them in stereo mode (which is odd since it isn’t supposed to be supported yet). Anyways, I said yes and they were added to a group in the Apple Home app.
What I noticed immediately is that if I tell Siri to play a song, she will only play the song in one of the two (again, probably due to lack of stereo support). If I go into iTunes and select both speakers (as seen in one of the screenshots below), I can tell Siri to change the volume and the changes will apply to both.