I just offset the aileron servo arms one tooth toward the leading edge of the wing and lengthen the pushrods so that the ailerons are centered. Or, you can move the aileron control horns back from the hinge line a little and that will also create differential throw. Creating differential by altering the linkage geometry eliminates the need to use two channels and doesn't decrease the throw of the servos in the downward direction which decreases the resolution of the system.
