The dual servo idea is not a bad one but you really need to be anal about matching up those servos. They have to travel the same distance and at the same speed. The least difference and your going to have a plane that will be very difficult to control.
Even with the enlarged control surfaces a decent single servo will cost you no more than 2 less expensive servos and will handle the job just fine without the headaches of trying to match up dual servos and be much more reliable. My rule in KISS it. Keep it simple.