Worth mentioning, another way of creating some mechanical differential for a single servo driving the ailerons would be to cut servo horns as shown in this picture.

In this case - looking from the bottom of the wing on a top wing plane the two remaining horns would be toward the front of the airplane. As the servo travels (in degrees) it would create more push (in distance traveled) therefor causing more up travel in the ailerons than down.

Needless to say, one thing to watch for on these setups is hitting the bulkhead in the fuse that generally sits right behind the wing saddle.