I have dual aileron servos, want to use separate channels (no particular reason other than to save a Y cable). Do I use P.Mix? I tried that, and it works fine, but when I set the trim, only the aileron on channel 1 moves. Is that right?

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.

The 6exa has support for flapperons which is exacltly what you want to do... Don't try it with p-mix... Enable the flapperons... Then your trim will work fine...