But the issue here is not smart home devices, rather multi-room music. I have the identical problem. According to Amazon (on their Alexa help page under multi-room music) it says:
“To stream different music on multiple devices or on MORE THAN ONE MULTI-ROOM MUSIC GROUP AT THE SAME TIME, you must have an Amazon Music Unlimited Family plan.” (the caps are mine for emphasis). So despite having Spotify Premium and Tidal, I’m now forking over for the macdaddy Amazon music plan, and still can’t have an upstairs group, downstairs group, plus an everywhere group that combines the two. Frustrating!
I haven’t tried telling Alexa to play music “upstairs and downstairs”. Awkward, but it may work. (That idea just came to me while typing this post.)