REloadRon made a very good and VERY IMPORTANT comment back in post #5. A fire suppression system is a LIFE CRITICAL SYSTEM and thus it is not a place to be adding any circuitry that is not approved by the equipment manufacturers. I would start with those diodes, 1N4007, rated 1000PIV and only one amp forward current.Wat to low a current rating for a life criticl system.

And certainly a fire system is no place for diodes across coils, because those are supervised circuits, meaning that there is always a small current running in one direction to monitor wiring integrity and in the opposite direction for alarm signalling. So your inverse diode is a short circuit in the test mode. I realize this makes it a lot more complex, but better to get it from me than some grouchy fire system inspector..

There is probably a correct and working way to have two systems drive one solenoid valve, but it is not as shown. There are solenoid valves with dual coils, or it may need two valves in parallel.

 

Having one FACP setting off the other is a really good idea.

 

Having one FACP setting off the other is a really good idea.

In addition, it may also be something that is a verified portion of the approved design. Many years back I was asked to add a set of door-hold-open electromagnets to a fire alarm system to release two fire doors when the alarm sounded. That is when I discovered the supervising voltage, which sensed the release relays and decided that they were a fault in the system. So I added a series diode to each relay connection to block the supervising voltage from the relays. Evidently it is OK, because the system has been passing the yearly checks for many years now.