Thank you @DaveC for the quick reply!
I thought about a hard mechanical switchover. I found a number of items searching for “phoenix connector” and “phoenix connector switchable”. Did you use a multi-bank type connector like this and run jumpers manually? https://www.phoenixcontact.com/en-us/products/terminal-blocks
Some further searching yesterday found this product also:
https://easyirrigationrepair.com/isolator
with some videos showing how to place it to isolate either a master valve or the common lead of a set of solenoids.
It *seems* as if I might be able to do both (with two units) in this application, as the isolator would likely interrupt flow back through the 2nd controller, but I’ll have to sit down with some paper and circuit diagrams and think a bit further first.
Ray’s answer in that other thread makes sense, from general principles. Still would give me the willies… if there were a programming error, I’d guess you’d get either 0 or 48VAC depending on whether the two sources were in or out of phase (assuming two separate transformers on the same AC main).
David