Confusing answer to using multiple servo for one surface:
(but... gotta cover worst case FIRST!)

There's a potential problem with running dual servos for a single control surface... the servo's postion sensing pots arent perfect, and can have the 2 servos fighting each other. Also, ANY mismatch in the control linkages will result in the servos fighting each other more and more as they deviate from the SINGLE position where thay don't have a problem..
This can strip servo gears and/or QUICKLY drain the RX battery.

There are ways around this.. spring loading the secondary servo's pushrod(s) with an R/C car sterring "servo Saver" would prevnet you from stripping gears. (but will not eliminate the high battery drain...) Futaba has just released a "match-box" which aids in matching the centering and travel of 2 to 4 servos. (I'm sure there's more...)

You can go so far as to modify a pair of servos to have both depend on ONE's positioning pot... (gut one servo and jumper wire form the other's motor to the one with no positioning circuitry.) There's a potential for problems with this solution: The "servo" with no position sensing circuitry will NOT try to hold position if its hooked to a different control horn than the other. (if the "control servo" doesn't sense the deviation... its allowed to happen.) This also means a long flexible control surface can twist, and can accumulate a significant twist over several throw cycles.

How much of a probem is normally seen when "ganging servos? Not much. Its commonly done for 1/3 scale and larger IMAC models. You just need to be aware of the potential probems, how to recognize if the problem(s) is(are) occurring and how to work around it.
Dual servos (I've seen "gang sets" of up to 5 for one rudder...) will work if set up properly.