Photopilot:

Nano servos are just small servos. Most of the nano size are smaller than a quarter. The ailerons require two servos because of the aileron location on the wing such that the two servos are an easier installation than one servo and the required linkage and less prone to damage. The servos are not very expensive so don't worry about the cost. You can run them both with a "y" harness, or if you have a computer radio from separate channels. That would make adjusting throws and and centering easier.
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EJ