You can use a reversing Y cable which will reverse one side of the Y. Available from MPI - Max products. The problem with this set up is that each servo may not move at the exact same speed or the exact same degree of throw on one side or the other. Sometimes you can adjust the programming of digital servos to even things up. Another problem is that long wire runs sometimes cause problems, ether because of voltage drop or the introduction of interference.

If you have a computer radio that supports dual elevator mixing then the problem is much easier and depending on the radio you can adjust throw and speed directly from the transmitter so the servos operate exactly the same.

Hopefully some of the giant scale guys will jump in and offer more specific help.