I’m not sure if you’ve made any progress here, but just to add a bit of info and clarification and to describe what I think you will want to achieve in the end, here is what I can offer:
There is no standard definition of a guest network in terms of the specific implementation details. The title of “guest” does not, in itself, preclude normal functioning of the Sonos.
The Sonos requires an internet connection and a non-client-isolated WiFi network. Any network that has WiFi client isolation enabled will likely break the Sonos functionality.
In your specific case, you said you want two independent networks with non-overlapping Sonos systems. To do this, you’ll want to set up 2 VLANs on your router that are isolated from each other. Each vlan will have its own SSID (WiFi network name), but will not isolate the clients on the respective networks. Both VLANs will have firewall rules setup that allow access to the internet and prohibit inter-vlan connections. You’ll probably also want to limit the access to the router itself from the secondary network (so that your guests can not alter the config of the router) - reject all router bound connections except for DHCP and dns.
That should be all you need and everything should work as expected - 2 networks isolated from each other, both with unrestricted connectivity between devices on the same respective network, and both able to access the internet.
Does this match what you are trying to do? And do you have specific questions about how to implement this configuration?