@DLane, thanks for the info.
I looked up the posts in which you shared your solution -
Run Multiple Zones At Once and Master Valve only on specific zones, and I get the idea of using relays to get the desired effect.
The point of the water hammer option raising the valve count to 3 is interesting, I haven’t thought of that. Thanks for sharing. I suppose one could consider putting a relay with a delay option (e.g. 5 sec delay) and then you could use the Rachio as the power source. Otherwise you would need your relay-powered zone to use external power to stay within the Rachio’s power limitations, right?
Another option that I’m considering is software based. Since the Rachio has API that can both notify you on Zone status changes and allow Zone control, I could possibly have a service (or IFTTT) that would listen to specific “Zone Start” notifications (in my case Zone 4 or 5), wait 5 sec (same reason as above, to stay within the 3-valve max limit), then open Zone 3 via API. Upon “Zone End” notifications for Zone 4/5, the operation is reversed and Zone 3 is closed via API.
I still like the relay approach a bit more, as it’s more solid than software, but I may still start with the API just for the fun of it.