I don’t have any grand solution. @controlav may have some insight into using a Raspberry Pi to bond the two systems together. At best this would be a research project and there would be some time align issues between the two systems if you attempt to simulate a Group across the two systems.

If you have a PORT and can connect its Line-Out to a Line-In on the “other” system, at least both systems can play the same content, but there will be a slight, consistent time skew between the two systems. This time skew would be significant in locations where you can simultaneously hear both systems, but would not likely be significant if the “other” system is outside and the remainder is inside the house or a garage, etc.

I know of no way to bond two systems together. You can of course have two independent Households on the same network, which would boost the total max devices to 64. You could not cross-play between them without a third-party solution (or line-in as buzz suggested), and synchronized playback between the two households would be impossible. No idea how Amazon handles multiple systems these days, but if the home is large enough for 64 speakers you can probably use two Amazon accounts, one for each “half”.

Ya, that’s been about what I’ve read to date.  I suppose the main purpose to posting on here is see how many others have hit this wall as well as we see more and more homes move to these setups and away from wired with the pandemic.  Home entertainment is booming right now and we see that in how many prefer to stream at home than go to the movies anymore.  I am one of those types, and have for the most part been very pleased with how Sonos operates.   

Wouldn’t be a big issue for me to set up my ‘home theater’ rooms on a separate ‘account’. I rarely use them for music purposes, if I were to run into this as a concern. There are plenty of speakers that can be used for music only. 

I have some ideas, if only I could find some free time. This would not be a world class integrated  UI, but it shouldn’t be too hard to brew a ‘CrossConnect” that is similar to a “Group”. This would not be integrated into a SONOS Controller, but would be a second or third controller. The first two controllers would be the SONOS system controllers for each system and then there would be the CrossConnect controller. Integrating Play/Pause/Volume with CrossConnect will probably not be too hard, Fully integrating music service control would probably not be practical because of the constant updates by SONOS and 3rd parties. Our poor CrossConnect would always be a generation behind and not have easy access to metadata.