I am also trying to do this – without luck. Apparently it should be possible if you do the set-up in Google Home. Tips and tricks very much appreciated.
M3 allows stereo pairing in the Google Home app and that will only work strictly within the Google Home/Chromecast multiroom system.
That means that the stereo pair is “”virtual”” and only available if you Chromecast to it via a Chromecast -enabled app. If you use any other way (Beolink, 3.5mm input, AirPlay, etc) it won’t work as a stereo pair