AQ,
The 662 has the exact same issue - the mixes do not support trims. Only higher end radios do this.

The flaperon function is a fully built in function, with proper performance on all levels including trims. The elevon and v-tail functions are also complete functions. You're asking to create a mix that simulates a full blown function, and entry level computer radios simply don't support this.

Doing a dual elevator setup is an example of where you might want to use a mix, and it DOES work, but it requires actual proper geometric setup and not using the trims, which so many modelers are now so dependent upon the trims that we once didn't have at all that we don't recommend it for fear someone will make this error.

The 6X manual clearly covers how to create a mix, but its up to the user to know what s/he is trying to do with it.