For users who have more than 12 zones, we are thinking about recognizing additional controllers as extensions, not as different units. When adding a second controller, it becomes zones 13-24, the third would be 25-36, etc. In this way, there will be no overlapping of watering times and users can see one schedule for the day and do not need to switch between controllers.
I think one possible workaround it to add restrictions, for example, you can tell one controller to only water on even days and the other one to water on odd days. Or one controller to only water before 7am and the other one only water after 7am.
Please let us know if you have any questions.