As Minnflyer said, Y harnesses typically do NOT reverse one aileron. But there are Y's made that will reverse one servo.

Also as Minn so nicely illustrated, setting up mechanical aileron differential is possible, and not difficult. I usually have my student(s) set up the trainer this way.

Ailerons are USUALLY on the right hand stick (mode II).

While there are other reasons, the primary reason for using seperate servos for ailerons is so you can program in 'Flaperons' on your computer radio. (The ailerons work as both, ailerons and flaps.)
Dennis-