using separate sonos systems so legacy products are not disrupted

There isn’t a way, using Sonos software, to integrate the two operating systems.

I have no idea about third party software, I don’t use any. 

You can set up an S1 and an S2 system, without them interfering with each other. Use the Sonos S1 app to control the S1 system and the Sonos app to control the S2 system. There are third party apps that will see both systems at the same time, but you can’t group S1 & S2 devices together.

Start here

https://support.sonos.com/en-us/article/known-limitations-with-separate-s1-and-s2-sonos-systems

https://support.sonos.com/en-us/article/set-up-separate-s1-and-s2-sonos-systems

https://support.sonos.com/en-us/article/set-up-a-separate-sonos-s1-system