Ken:

A single aileron servo will work on almost any airplane in the 0.40-0.60 size range, but usually two are used. Couple reasons. There is the possilility of overloading a single with aerodynamic forces, a biplane seldom is fast enough to have a problem here. Two servos just seem to flip the flaps faster. More precise control from having less linkage pivots. And for the same reason less tendency to flutter.

Comes down to you - add the weight and expense, go cheap and adequste.

Bill.