With the Y cable you put the control linkage on opposite sides of the servo's to get the opposite movement effect. They both turn the same way but one is pushing & the other is pulling on the linkage.
As for dual channel servo's you have to have a computer radio that can mix the channels or one that has a prebuilt Aileron Mix. You essentially slave one servo off of the other, so that when one move's the computer makes the other one move too.

One advantage of the dual servo's with the computer radio is that you can do things such as flaperons & more advanced mixes.
I'm not sure if there's any negatives with the Y-cable other than that.

Happy New Year to you too