I want to take my RainMachine (in my case an Touch HD-16), and run it "in parallel" with another sprinkler controller. In detail, that means they will both get the same 24V AC input, and some of the valve outputs will be then fed by both controllers. Obviously, I'll make sure the polarity of the input is correct, so when both controllers turn out, there won't be a short. In a nutshell, it amounts to the same as taking a piece of wire and touching both the "hot" side of the 24V AC input and a valve output, to manually turn on a valve.

Will this do something bad to my RainMachine? I hope not. I don't care that I'm voiding the warranty or doing something unsupported, I just want it to work, also in the long term.

If this won't work, then I have to modify the "other" controller to have SPDT relays, and disconnect the RainMachine when it has to turn on. That's going to be a big pain (a lot of extra wiring, and a half dozen extra small relays).

Why am I doing this? Because I have about half a dozen sprinkler valves that need to be turned on simultaneously (at least 2 or 3 of them) in emergency situations (fire suppression sprinklers), but are also used one at a time in normal operation (irrigating the landscape). The RainMachine can not do multiple sprinklers at once, but it is great at doing normal irrigation. So my cunning plan is to connect it in parallel with the emergency sprinkler controller (currently a box with toggle switches, soon to be an OpenSprinkler with a Raspberry Pi and modified software).