RYGUY,
Sorry, but you are incorrect about the 6XAS not being able to mix. I just did a mix on mine for KE pitch coupling, but in order to accomplish this, you need to asign the aileron, elevator and rudder dual rates to the Ail D/R switch. In doing so, you free up the Elev. D/R switch for 2 more programable mixes, PMx2 and PMx1.
BACKWING,
If you have the 6XAS manual( mine is fairly old so I'm not sure if the page #'s will match yours ), page 39 details combining the Dual Rate switches, and pages 35 & 36 detail the programable mixes. I also have a good link to the Futaba FAQ section for the 6XAS.
http://www.futabarc.com/faq/faq-6xsuper.html
It takes a little experimentation to get the mix just right, i.e. right amount of elevator input, but it can be done. Believe me, if I can do it, you should have no problem. If you have any questions, just reply and I'll run through it with you as best I can.
Longaly
p.s.
With the mix being assigned to a switch, it is selectable as in ON or OFF. Also, my aircraft was pushing HARD to the belly, yet my mix percentage was quite low, -7 & +7. Make sure you move the rudder stick full in each direction to see and change the percentage of mix. I hope this doesn't confuse you, I was at first, but look at the manual and the above referenced web site and all questions should be answered.
p.s.s.
FOLKER is right, a properly set up aircraft should take care of most coupling problems. Get with a pattern guy in your club and he will be able to help you with the sometimes tedious and lenghty process of properly triming your plane.