Actually, you can't fix the difficulty. If you have a second 6X transmitter, then you'd turn on its elevon mixer.

The difficulty is that the 6X-series of transmitters doesn't have a complex-enough processor to allow you to use a function instead of a channel. It just takes the raw signal output of the other transmitter and sends it to the model. If the 6X is being used as the instructor's transmitter, and has mixers turned on, and the student's transmitter is less complex, and/or doesn't have mixers, then the mixers in the 6X can't be used when the student's transmitter is operating.