Hi Franz, we had to face a very similar problem on a security solution. Customers wanted it, we built it, and we could not keep state on two boxes synched fast enough. Needless to say, nobody used that feature. However, since customer pressure was high, we eventually solved it - an entirely different way. We divided the box into control and data planes. We could extent the data plane over multiple chassis, but the merged data plane would only be controlled by one box. That worked amazingly well.
In your application, you could make the second controller a simple port expander to the first, but the intelligence would run on the first, only now with 16 (8+8), 24, or 32 zones. You could run something as simple as SSL/SSH link between the two boxes. One box would become a passive zombie and obey zone calls from the first box. The software already knows how to deal with 16 zones. It cannot be that much more difficult to add 8 or 16 more. You would have to develop a simple control interface between the two. Of course, a HW solution for gen 4 and some serial interconnect which automatically puts the secondary box in receiver mode, is also feasible.
The question for Rachio would be can you guys grow faster in the do-it-yourself small homeowner market by adding more features and beating the Hunters and Rain Birds, or do you go into commercial, become a supplier to contractors in which case zone expansion is a must. Of course, there is much more to it, like understanding the value chain or creating some form of managed service model. In any case, I would not underestimate the need of more zones, like us folks in TX which sky high water bills, relatively large properties, and hence many (often micro) zones.
I am happy to chat, if you want to brainstorm. Have been a product manager/designer/architect for many years.