Yes, you should be able to run both in the same LAN. 

I’d recommend powering down all Sonos S1 devices when setting up your S2 devices, but after setup, power them back up, things will work fine. 

That would take me a month to transition that way… I refuse to switch to S2… it’s just too difficult. I have a few gen 1’s that are not S2 compatible yet all of my current devices are S1 compatible. There may be some benefits to upgrading to S2 but i will not sacrifice the Gen 1’s that aren’t compatible…. And to toggle back and forth is ridiculous to me. Unfortunately i wont continue with Sonos… I’ll stick with what i have. I’d love to purchase the Roam… a smaller portable Bluetooth/Wireless speaker but wont do it. For that I’ll shop elsewhere. 

I’m curious. Have you never upgraded / replaced a mobile phone, a tablet, or a desktop computer? 

Yes I have. Lets hear it…

Power down all of the S1 units and force close the S1 controllers. Next, setup a new S2 system with your ROAM. Finally, power up the S1 units. You will then have two independent systems S1 and S2.

Thanks Buzz… I do appreciate your advice. But for me - that is sooo much easier said than done. It’s not you that I don’t trust. Ive tried twice to establish both connections. The most recent was catastrophic. 27 days without my entire system. 4 days to find someone from Sonos to contact me and 11 hours… multiple diagnostics and 11 hours of phone dialogue with Sonos walking through “remedies”. It was a NIGHTMARE. I can’t go through that again. 

You shouldn’t need to do this again.

Another plan would be to take the ROAM to a house that has no SONOS system and setup the ROAM as a “new system”. You can use the same email address as the S1 system. When you return home you can update the ROAM’s WiFi.