Need to set a parameter value using a TX switch

I’m sure this is possible but I can’t figure it out myself, so experts please help.

I just want to use a switch on my TX (X9D) to set the RUDD_DT_GAIN (Differential Throttle) parameter value while in flight to either 0, or 30. I have a 2.4m twin engine scale flying boat and I need RUDD_DT_GAIN=30 on the water to stop it driving around like a headless chicken but I don’t want differential throttle while in the air.

I can easily mix two throttle channels on the TX to accomplish this, but because I am using Left and Right (73/74) throttle outputs to the motors, the TX setting is ignored. Changing the servo outputs to straight-through works nicely in manual mode but I know this is a really bad idea for Auto and RTL modes as the Pixhawk won’t know about the 2nd motor.

So I am pretty sure I need to stick with 73/74 and find a way to change the Diff Throttle parameter from the TX.