Depends on the streaming service, whether they allow two independent streams. Which service are you using?
Spotify premium family package
Have you tried deleting Spotify from the Sonos controller, waiting five minutes, then reinstalling it? The Sonos system should be recognizing the family subscription, and allow two different streams, perhaps forcing a new ‘handshake’ with Spotify may fix that.
If not, I would recommend that you submit a system diagnostic within 10 minutes of experiencing this problem, and call Sonos Support to discuss it.
There may be information included in the diagnostic that will help Sonos pinpoint the issue and help you find a solution.
When you speak directly to the phone folks, they have tools at their disposal that will allow them to give you advice specific to your Sonos system and network.