My house has two separate irrigation controllers mainly due to proximity to the valves and wiring. I however view my system as one "unit". Assuming these controllers are on the same physical network I would like to see things like...
* zone schedule not conflicting between controllers
* utilizing a single master valve connected to one controller to also work with the other controller
* combined reporting
One idea would be to identify a single controller as a "master" (possibly the one with the master value attached) and the others as slaves. The software could then treat the zones as a single entity with the scheduling handled by the master and just utilizing the slaves to turn the zones off/on. Utilize notifications to handle situations where a master or slave is lost.