jatt5abdaa,

You can use one servo for both elevator halves, I have done this on a few planes.
For it to work properly the servo MUST lay on it's side, and the servo arm needs to be centered in the fuselage.
Even if the servo (mounted upright) were centered in the fuse you could not get it to work properly. There would be a lot of slack in the cables when the elevators were deflected.
You also need to have the control horn/hinge alignment as close as possible.

Bill