SUCCESS!!! A hearty thank you to @DLane for his schematic and sourcing the required relay.
A backyard zone now has an extra valve inline so that I can sprinkle just my garden or the garden and the much larger area beyond. @DLane and @Emil stepped up and through their guidance I got around the current inability for the Rachio controller to power 2 different zones at the same time.
I bought this 24VAC relay from Zoro for $15.25 all in and it arrived 2 days later.
@Dlane, let me know when you are in Austin and I will buy you a beer, or better yet I'll make you a salad with vegetables from the garden.
Thanks for all the help!
P.S. My hardest part was realizing that the Master Valve setting is enabled through the web interface and not the mobile app.