RE: Multiple servos for elevator?
The arms have to be on the same side of the servos for elevators using aft side mounted servos. The geometry to the horn on the surface is different for each side if the arms are pointing oppositely. There will be a differential effect between the elevator halves.
With the servos mounted in the sides of the fuselage (rotation axis horizontal). one must have the electronics reversed.
If the servos can be mounted normally (rotation axis upright) in the cabin, then the pushrods can be attached to the opposite sides of each servo and the motion at the elevators won't have the differential.