This has been brought up more than once in the past. I suggest doing so only if you have a good idea how the other controller works internally. The reason is that not all controllers work in the same way. On OpenSprinkler, one wire of 24VAC is tied to the circuit ground, and the other wire is common. For each solenoid, one wire is tied to common, and the other is tied to a triac. Switching the solenoid is done by turning on the triac thus completing the circuit. On other controllers, the circuit ground may be the common wire; also many controllers use an isolated design. Without knowing the underpinnings, it’s difficult to tell whether a SPDT to switch between the common wire is sufficient to get them running safely in parallel.