I really dislike these “ISOLATORS”, a better isolation can be achieved with one double pole relay like this $8 relay from amazon (link), more on such setup here (link). @DLane has outlined the steps for such a setup with a similar relay (exact relay does not matter for this setup). One thing to note is that NO terminal should be connected to 24 -VAC terminal in step C of @DLane’s post, this is counter intuitive to how things are normally done so…
Back to the topic at hand. Try connecting the black (input) wires from each of the isolator to the M terminal of the controller nearest that isolator, then connect the red (output) wire out of each isolator together with each other and the MV wire going out to the field, finally hook up all of the white commons (four from your isolators, two from both of your controllers and one from the field wiring) together and see if your system is now working.
Edit: Just noticed that you’ve actually started this tread with a nice diagram of your setup. The problem is that these “Isolators” will also disconnect the common so the controllers will not be able to water anything at the same time. Here is an updated diagram to help you better understand what needs to be done:
Cheers,
Gene