If you have the X9303 then I would use the radio to do it. Reversing Y-harnesses have a habit of the centers drifting on the servos. The advantage of using the radio is that you can adjust each servo to trim them in using only the radio. Trust me, you'll be much better off using the radio over the Y-harness. Plus, it will save you the money of not having to buy the Y-harness.

Ken