ORIGINAL: LargeScale88
I would adjust the clevis or rod on one servo to match the other. Sense one has more movement then the other, adjust where the clevis hooks on the servo arm, or shorten or lengthen the road going from the servo to the aileron. I've got my computer radio which does the travel adjustment for me, (not sure if yours is computer), anyway, just adjust the clevis and rod.
That. Get them approximately centered with the servo arms and then fine-tune by turning/adjusting the clevis connectors. Once they are centered the throw will be equal enough from a "Y" connector set-up.