If your servo horns are setup right, ie.. both horns are to the outside of the wing or visa versa.
I am sure RCKen has a drawing ready to post.
In most cases a Y harness works perfectly, on the off chance your setup is different (not common) there is a reversing Y harness but you probably will only need a standard one like this
http://www2.towerhobbies.com/cgi-bin...?&I=LXWB69&P=7
Take a look at your servos, if you turn both of them clockwise what happens? In most airplanes both servos turn in the same direction but one aileron will move up and the other will move down.