Thanks Gene for the awesome explanation! I had to go figure out what you meant by “complete isolation with two controllers is not possible” and that lead me to my final drawing 
I never realized the valve solenoids are just a coiled continuous wire and like you said, the 2 controllers can thus never be completely isolated (as my drawing demonstrates).
My new drawing includes my plan to change the master valve control relay. Now I can have both controllers running a valve at the same time and only one controller will supply power to the master valve. Blue line is current flows whenever Controller A is running, Solid Purple is current flow when a valve on Controller B is running (doesn’t matter if Controller A has a valve running), and the dashed purple is current flow to master valve when Controller B is the only controller running a valve.
Thanks again for spending so much time on this thread!
Updated diagram per Gene’s below comment
