Good questions
I think I can answer them.

On smaller planes you can run Both Ailerons off of one Servo.
In fact every trainer I have seen has been set up this way.
However this is not practical on larger planes or on planes where the ailerons are far out on the wing. A Mustang for instance.

on a 3D plane or a .60 size and up plane, you want the maximum speed and reliablility possible. This means using a servo for each control surface. And in some cases you will even have two or more servos per control surface.

You can run both Aileron servos off one channel with the use of a Y harness. These cost about 10 bucks and are exactly what they sound like. They split the one input into two. One Servo will also be reversed off of a Y harness. Very important for Ailerons

Now I prefer two aileron Servos and two channels. My reson for thisis simple. You have much more percise control over each aileron. You can make minor trim adjustments to one without having an impact on the other. And you can mix them to move up and down at different rates or with more or less travel independently. Something that can't be done with a Y Harness or single servo setup.

All computer Radios are set up to allow you to run two aileron Servos. In the case of my 9C it is channels 1 and 6. ALthough I believe you can switch the second one to channel 7 instead(or maybe 5) Either way there you go. I hope that helped.


Opps

Theoreticly you could use a smaller Servo for the Ailerons if you used one servo per. However this isn't common practice except in thin winged planes. For instance my .40 Sized Pits has a wing that is to thin for a standard servo. So I use Hitech HS81s They are smaller but offer the same ammount of power as most full sized.