There's no extra servos needed but you would have to cut into the wing to allow installing pushrods from a central aileron servo. The other servo would become the dedicated elevator servo. So still only two servos.

But yeah, the easy way on a wing which is already built is to put two smaller servos out in the wings in line with the separated outboard ailerons and one in the middel to run the separate elevators. If the elevators are split due to a sweep angle at the center line you can run with the existing two "elevon" horns and just make up a solidly joined Y shape pushrod to run the elevators.

The other alternative is to stick with elevons but move the two servos out onto the outer wing positions so that the servos have a straight line to the control horns on the new half size outboard elevons. So again only two servos are used but you relocate them from the centerline out to positions mid span on the wings. With the new 40'ish % span elevons the roll response will be very close to the same as before with the same deflection angle but the elevator response should be cut down a lot by the reduction in area.