I've been going at this for probably an hour but can't seem to figure out how to connect more than one speaker to the box. It does seem completely insane to me that it wouldn't be possible to do since they're from the same company and it seems like the most basic thing ever to connect TV with speakers.
Nope, there is simply no way as of yet! :-( I'm hoping for a feature like that for ages. It's said, that US beta can group Android TV and smart speakers, but i didn't succeed yet. Can't believe that Google is blocking such feature, as Apple can stream audio from AppleTV to homepods... But we are talking about Google! :-(
You should ask the miibox subreddit.