I would be careful putting two controllers in parallel. The reason is that the internal mechanism of how each controller switches a solenoid can be different, and if both try to turn on the same solenoid at the same time it might result in shorting. I think if the following two conditions are satisfied it should be safe:
1. each controller uses its own power supply, no sharing of the same power supply
2. it’s guaranteed that the two controllers won’t turn on the same solenoid at the same time.