I If the S1 App already now controls both Homes, it will continue to work in your Home that has the S1 system, post upgrade to S2. I’m guessing it’s likely a different Household ID anyway, but if you do find it doesn’t work, for any reason, you can always reset the S1 App anyway (which is only a ‘remote’ for controlling the speakers) and on reopening, select to connect it back to the existing S1 Sonos system present on the connected WiFi network.
You can install both controllers on a phone/pad/computer. In the soon to be updated home, simply update the system to S2. If the S1 controller is not uninstalled as part of the update, uninstall it after the update. On return to the S1 home, force close the S2 controller, install a fresh copy of the S1 controller and make sure to join an existing system, not setup a new system. All critical details are stored in the players, not the controllers. SONOS Playlists will not be shared between the two homes, but some services, such as Spotify, will be aware of activity on the ‘other’ system.