You don'y use one servo for up and one for down...Never seen it that way. Two servos are used for a SPLIT elevator where the right and left elevator halfs can move independently, Primarily for higher torque. The servos are placed outside at the rear of the fusalage and short control rods connect to the elevator horns on the bottom, pushing and pulling. You can "y" them with a harness or use seperate channels for each, and program your radio to move them in unision, but you can also Mix them on the radio for special manouvers like a REALLY tight roll where the elevators mimic the airlons... one up and one down.
The only other way to use 2 servos is to "gang" them which is like running two batterys in parallel, typically inside the fusalage, and only on giant scale . Hope this helps
Mike