Many of the planes today have dual aileron servos on them and many will tell you that it improves performance. But that's not always true. In fact, one of the best flying planes EVER made uses only a single aileron servo mounted in the center of the wing. And it doesn't suffer performance at all. If you're interested in what plane I am talking about I'm referring to the Great Planes Ultra Sport, both 40 and 60 size. Back when planes like this were designed servos were very expensive and using two servos for ailerons were unheard of. These days servos are cheap and it's easy to have 2 aileron servos. So if it's so easy to use two aileron servos why did I even mention this? To point out that it's not necessarily an improvement in performance that would dictate 2 servos, because if the plane is designed right one servo will suffice. Now with that said, there are good reasons to have 2 servos as opposed to one. A few worth mentioning. Servo failure. With 2 servos a blown servo on one aileron will not kill the plane as you can still land with just one aileron. With 1 servo there are usually a torque rod setup which might result in flexing of the aileron in extreme manuevers such as 3D flying, where the short pushrods from a servo on each wing results in less flex. Also, the shorter pushrods and the geometry of the servo in the wing gives the control surface more power to move the control surface. These are just a few of the reasons why.

To answer the second part of your question. With a Y-harness both servos receive the same instructions at the same time, therefore they will both move in the same direction. Refer to the attached diagram to see what I am talking about here.

Hope this helps

Ken