In post #2 there is a link to a picture of QQ's set up. I notice that he uses a wheel/pulley on the rudder servo, but not on the elevator servo. (I assume that it is not the other way around.)
However, the reason that you get a slight rolling coupling with a vertically mounted elevator servo is that you have a sideways movement of the servo arm as this moves away from neutral, thus pulling more on one cable than the other.
Would not this be removed if you used a wheel (pulley) on the elevator servo, keeping both cables equally tight?

Magne