You use one servo in the center of the wing with control rods running to the retracts. You have to use a retract servo because they have more throw, or travel than a standard servo. There are a number of brands available; try "Servo City". Retract servos are on the expensive side at around $40 is the cheapest I've seen. (But I haven't been looking.)

The hardest part of setting them up is usually getting the geometry of the cntrol rods right, so that the retracts travel to both full down and full up, without the servo binding.
Dennis-