Reasons for 2 servos:
More power to each aileron (more torque).
Easier to adjust trim using radio, rather than messing with control linkages (if you have a computer radio).
Easier to install, uses standard control horns, control rods (instead of torque-rods which have to be built in at construction time).
Can play with flaperons, etc (if you have computer radio).
Typically have shorter control rods, reducing the amount of slop in the linkages.
Is "cooler" than one servo.

Reasons for 1 servo:
less weight
less cost.
Typically all that is needed (for most non-3D non giant-scale planes).
Mechanical coupling of ailerons is advantageous because you trim both at the same time.
no need for expensive computer radio
no need for 5 or more channels / y-harness.
If the instructions will call for this setup, it will most certaninly be adequate.
Simpler interms of trims, centering, and everything else.

Basically, if the Kit says "One servo", then the chances are good that is all you need. If you need two servo's for you flying style, then you are probably exceeding the planes capabilities in other respects.

If the instructions call for 2 servos, then it is probably because the additional control sensitivity (less slop, more torque, shorter rods) is required to suit the flying style of the plane.

There is no need to have a computer radio, a Y-Harness will work just as well.

This does not answer your question directly, other than to say that there is more reasons than just "to have flapperons" that you want one servo per aileron.

gus