I’m working on solving this problem as well. I have two separate irrigation systems in front and back where the back is the system controlled by the Rachio 3. I’ve purchased components to control this with Raspberry Pi wireless controller cards and voltage sensors and relays.
I plan to put a voltage detector on Zone 4 and program the Pi to watch for the Zone to activate. It will then communicate to another Pi attached to my front zone where I plan to use a relay to turn on the solenoid and the irrigation to the front zone. So far, instead of costing $450 like the Hunter system, I’ve got about $110 invested.
I hope to be able to report success in a couple of weeks.