In any case, if you have easy access to the individual ECU to receiver connectors, you can disconnect and reconnect the engine that did not start and retry to start without having to shut down the other one. (If the propane got too cold, it will not start anyway)
On the 12X (I guess on any 12 ch tmtr) you can program to start each engine individually. By using a three position switch you can select either one, or both, holding in idle the non selected one. Throttle trim and stick will be active for the selected one only.
This is a complex program and uses 3 channels if I recall correct. So, unplugging and replugging the ECU connector is a lot simpler.

Jack