@kcheek - The relay can be by the Rachio. That will be shorter distance with all the wires involved. The distance isn’t a problem, just like a solenoid on a sprinkler valve.
While PM’ing with @Gene on this he figured that one Single Pole Double Throw (SPDT) relay would also do the trick instead of two Single Pole Singe Throw (SPST) relays. In this configuration, M (Master Valve/Pump) terminal from the Rachio would need to be connected to NC (Normally Closed) terminal, SC (Sensor Common) terminal from the Rachio to NO (Normally Open) terminal and the pump relay feed-line connected to COM terminal of the SPDT relay, control coil would be driven directly by the Hunter controller (Com on one side and P/MV on the other side). In this configuration the pump start relay would be controlled by the Rachio M terminal as circuit is normally connected. When the Hunter controller requests water it will swing the relay to the NO side and to power the pump start relay using the Rachio SC power. In this configuration there would be a slight break (milliseconds) in signal power to the pump start relay if both controllers were calling for water as the relay switched from the NC to the NO position. I don’t think that should be a problem.
Now we’re probably really confused you with four different ways to solve the problem.