I'm kinda new at this, but I think you want both aileron servos on the same channel, so the y- harness in needed. A single channel for both wing servos.

In any case, you want the ailerons to work on the same channel period, otherwise you would not be able to control the plane at all.

I am sure I will be corrected if I am wrong, but I'm pretty sure of this.

Good Luck!!