I fly a UCD3D with a T6XAS, and had some of the same issues. I don't know of a way to set up independent elevator control with the T6XAS. I rewired one servo to run backwards, and used no other electronics. Even the slightest misalignment of one or both of the control horns will cause tracking errors at extreme throws. The row of holes in each of the two control horns must be aligned exactly over the hinge line. The other issue I had trouble with was servo centering. If the two servos are not centered exactly alike, they will not track, and, as per an old guy named Pythagoras, the problem will be worst at the control extremes due to the nature of triangles. You can minimize the effect by using the last hole on the longest available control arms on the servos. I went through a lot of grief over this, but it all tracks well now.

Chris
Arvada, CO