I'm sure you've seen some plane have single servo to operate both side of the aileron per-wingspan and while other operated on two servo where one servo operate the left aileron and another one operate the right aileron per-wingspan.  Let examine the differences  between them.  Along the way do read some article right here (http://homepages.paradise.net.nz/bhabbott/Servo.html)  to get some necessary info before we go on for further understanding on servo operation.

 

SINGLE SERVO operation. (Two aileron left and right operated on single "centralized"servo)

Pros:

Cons:

img ttrlfpv servoload single ailerons

 

DUAL SERVO operation.  (Where each aileron have its own dedicated servo to control the movement)

Pros:

Cons:

img ttrlfpv servoload dual ailerons1

 

Simple Experiments
If you wanted to know how much load can your servo take on different (single/dual) servo setup on your wing try this:

  1. Turn on your plane as usual. Release your aileron stick to neutral.
  2. Now put your finger or thumbs under both ailerons at the same time both with equal pressure.
  3. Apply some pressure to simulate air/wind loading on aileron control surfaces.  You should hear your servo starts to jitters after few seconds as it receive some pressure load.  Try hold it on for 10~30 seconds.
  4. If you are lucky the servo will just get warm, if you're unlucky its either jammed due to coreless motor/MOSFET overheating heat or burned.
  5. Then release your finger or thumbs from the control surface (ailerons) to end the test.

Now you knew why you shouldn't have a heavy plane or payload especially if you have big aileron surface.

 

Conculsion
Actually both have their advantages that suits best on their individual flight application and requirement, we cannot assume everything is perfect since there are many grade of aftermarket/commercial servo available for consumer.  Single servo are good for long range, high stamina, cheap and low power consumption plane while dual (independent) best for temporal precision flight application such as 3D and aerobatics. A stiffer aileron hinge with auto return to neutral position springs works best to reduce wind turbulence load stress and the inability for certain servo to return to neutral "0" (Zero) degree after motor/MOSFET fail or burned.  But most important thing when considering buying a servo do check the following attributes before you check in those servo under the wings because it may save your neck and pro-long its lifespan.

Check list servo attributes:

Good luck.