I don't have a 12X but have done similar things with my 9303. If you do it as Craig suggests I don't think your trims would work properly. Try leaving it set up as you have and just mix aileron to elevator. You might need to use two mix functions, one for each elevator half. You could also use a mix switch to turn the feature on and off if you wish.

You'll have to play with it a bit. Good luck.

Joe