There is a similar post here with an incorrect accepted answer:
In fact, a speaker group containing stereo pairs can be streamed to. I can confirm that Pandora streams to one I have created just fine. The issue appears to be isolated to YT Music.
have managed to resolve this issue which involves resetting one pair of google nest audio speakers.
I tried starting playbacks on different groups of speakers, and found out a few groups which contained one pair of nest speakers would refuse to start playback.
I then narrowed it further down by removing unrelated devices, rinse and repeate a few times later I found the culprit.
To fix this issue, I first separated the nest audio pair, then deregistered the two nest audio speakers from my account, then performed reset (mute + hold middle button for 15 seconds), then re-register it to my account.
Thanks for sharing. I did see a similar reply of yours to another thread on the matter. It just seems like a convoluted solution to a problem that only seems to affect YouTube Music.
I have tested Pandora, Spotify, and Deezer, none of which require anything to be done to stream to a speaker group comprised of two or more stereo pairs other than simply selecting it.
Thanks again for helping here, @Johnharper1. We appreciate you posting your resolution in the Community forum. @TwilightCitizen, I hope you've got the answer you're looking for. In addition, you can check this link about stereo pair and speaker group.
If you're still in need of any assistance, feel free to let us know.