When we connected a second phone to our new Sonos Beam (first Sonos product), the second device did not require a login to the Sonos account, so it seems to me that simply being on the same network makes it consider all local Sonos devices as one.
One way to fix this is to partition the network -- create a second wifi network either on the current wireless access point (if it has multiple radios and supports this, but if you didn't buy it for that feature it probably doesn't) or by buying a separate wireless router to connect your devices to. In the latter case, you'd want to connect it to a wired ethernet port on the existing router, use a different SSID, and have it run NAT to provide local IP addresses -- don't run it in bridge mode. You'd then want to make sure all your other devices (phone, computer, etc.) are on your new wireless network, so they intercommunicate properly. If you have any shared devices in the home, though, this gets more complicated, whether you're both operating the same smart lights, sharing a file server, etc.
There's absolutely zero need to partition the network. Multiple Sonos systems ('households') happily co-exist on the same IP subnet.
@Sdolan7: Simply set up your speaker -- and its controller -- as a 'new system'. If the speaker and controller have already been configured to work with the first speaker you will need to
factory reset the new speaker first. Also reset the controller, since right now it's associated with your housemate's system.
Your controller will remain dedicated to your speaker, and your housemate's to his.
