At first I could see that GH knew I had another Chromecast but it didn’t look active. I then realized that the USB port on my receiver had died so I added a plug in. No matter what I say or do I can’t find the right command to get the kitchen GH to cast the same music to the living room. One thing that I did find was GH can cast two different songs at the same time. I use the living room Chromecast for that room and to feed an amp that feed four pair in wall speakers.
Multi-room playback (groups) is only possible with Google Home and Chromecast Audio. The HDMI Chromecast can not be added to a group.