Both David Cutler and Minnflyer's comments in this post are correct. I've never met either, but have sorta known them for over a year via RCU. The are both knowledgeable and offer good advice. Minnflyer pointed out a way to mechanically set up differential. But with two aileron servo's, it is much easier to do in my opinion by programming the radio, as David pointed out.

Swager: With a 2 channel aileron setup, you still control both servos with one stick on the transmitter. The radio just "knows" to run two servos from that stick. It "knows" because you program the radio as such. With a one servos setup, the radio "knows" just to run the one servo, again because of how you programmed the radio. Hope that helps.