When you use a pmix in the Futaba 6EXA, the trim only works on the channel that has trim. It doesn't carry through to the mix. Only the 9C and higher transmitters have a TRIM option in the mixers, and that may or may not be available, depending upon the mixing options chosen in other mixes.

For two aileron servos, use the Flaperon mix and just leave the flap travel at 0%. That's the way to do it, not a pmix.