With the barn door ailerons, rig the linkages to give twice as much upward defelction as downward. That will greatly reduce the adverse yaw tendency. With equal deflection, the aileron induced adverse yaw can overpower the rudder.

By chosing where in the rotation arc of the servo wheel the linkage connects for neutral, you determine the amount of differential throw. With the control horn on the bottom surface, the linkage should be forward of parallel with the spar. (or forward of vertical if the servo is on its side.) You can go as far as 50 deg forward before the surface will start going back up when you are trying to move it down. (at 90 deg, it pushes up either way you move the stick)