Montague is on the money. You cannot set aileron differential electronically with a Y harness. You can however set up the linkage as per Minnflyer's picture.

I first of all question why you need aileron differential in the first place. Its only function is to make rolls more axial (reduces adverse yaw). It is NOT a concern for the average Sunday flyer and at your stage of expertise, you probably couldn't tell the difference. No RC airplane needs aileron differential to fly with the possible exception of a J3 Cub.