Just to confirm, when you use the term ‘join two rooms’, you mean ‘grouping’, correct?

I’m not experiencing this, and am listening to a baseball game right now with several rooms ‘grouped’, so I may not be a bunch of help, but to troubleshoot, the first thing I’d do is reboot the router, and the speakers, and see if that made a difference. 

If that didn’t work, I’d 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 network and Sonos system.