@janderson, I see no problem with your plan (as long as your properly schedule your controllers to never be active at the same time), good job. My only feedback is that you could accomplish a similar setup using a single SPDT relay, more on this can be found in an old post (link). Only difference is that you would use R1 terminal on your Gen 1 controller, instead of SC terminal shown on the original wire diagram.
The fact that above post discusses a hunter controller does not matter (it is simply a second controller), pump relay is essentially the same as the master valve for the purposes of this discussion and using SPDT setup has an additional advantage of not having one of your rachio’s potentially driving three loads (two valves & one relay) at the same time.
P.S. @DLane is right, ideally the commons for the valves driven by a particular rachio would be separate from the other valves and connected directly to the corresponding controller. If this is not possible, sharing only one line between controllers (such as your commons) can work without creating a potentially hazardous circuit.