@Laura_Meier, nice dashboard, thanks for sharing and I like the bright yellow tiles on that background. I don’t have another echo with me to test at this moment, but based on the Echo Speaks document, you can add the music(audio) group in the search music command.
ex: searchAmazonMusic("thriller on the upstairs") where the ‘upstairs’ should be the multi-room music group. Let me know if this works. 
Update: @Laura_Meier I was able to test out the command and play music to multiple echo devices in my rule. See this post for the detailed instructions.