Sounds like a router issue as Sonos only connects to the SSID you enter in the controller. If the router internally joins the SSIDs this could happen, you might also see a similar issue if the SSIDs are joined to the Ethernet and you try to use that.

You might pick a quiet time and disable the tenant's SSID and set up your Sonos.

Thanks for the response. I was thinking the same thing! But then he will automatically link to mine when he tries to reconnect won’t he?

I wouldn’t think so. I am pretty sure it connects to the last system it was connected to, not the last one that was set up. So his controller would connect to his system (when it is powered back up) and your controller would connect to your system. 

I assume that you did not have an existing SONOS installation before you started the attempted install.

I suggest that you Factory Reset your units, uninstall your controllers, then keep your SONOS units powered down for the moment.

Next, have your tenant go to Settings → System → Network → Manage Networks and remove your SSID if present, Then, have your tenant power down all of his SONOS units and force close all of his controllers.

Now you can install your units as a new system using your SSID, then your tenant can power up. You may need to fuss a bit if your SONOS units insist on using the wrong SSID. After setup you can always go to Manage Networks, add your SSID, and remove the tenants SSID.

Two or more independent systems can exist on the same network, but you’ll likely have fewer future surprises if neither system has any units wired to the network and they use different SSID’s

Thanks all- I will try this approach