Cannot play multiple Spotify streams when one room is started via Google Assistant and the other via Sonos App

You can only play from one Spotify account at a time. So when you start to stream from the Spotify app, you have depleted your possibilities to stream to other speakers from that account. If you group Sonos speakers however they are seen as one, so you can hear Spotify everywhere. Possibly the way you start streaming with Google works the same as playing from Spotify itself.

Ok but how do you make the Sonos uses only one account for its Spotify integration, regardless of non voice or voice.

The goggle assistant simply connects to the one account that it finds.

I don’t understand why a voice initiated stream interferes with a non voice initiated stream.

A Sonos Household is exposed as a single device to Spotify, so different devices can play different things at the same time. However the Google voice integration knows nothing about your Sonos household, so is considered a different device, and will stop other devices from streaming.

Though it should be possible to start a stream by voice command on a Sonos speaker and then group that speaker with other Sonos speakers. Do you use voice commands via a Google device or via the Sonos speaker?