You have to input the gear "throw" amount into the radio, then make sure that the gear switch activates the servo.

You also must mix the gear channel to AUX1 at 100% ( in both directions) , and leave it set to be permanently "on"...

Then you must set the throw amount for AUX1 to ZERO in both directions!

The last point is very counter-intuitive but standard for JR radio software which the Spektrum uses.

The throw amount for the slave channel controls the amount of movement controlled by the aux1 knob or switch.

You DO NOT want the one flap to be accidentally bumped or moved in response to the AUX1 knob or switch, so you must set the amounts to ZERO.

Instead because the AUX1 channel is slaved to gear, AUX1's throw becomes controlled by the throws, movement, and trim of the gear channel.

I used the above arrangement on another plane recently w/o problems.