Double check the orientation of the servo horns. That is, are the horns both on the "right or left" side of the servos? if so, set them so they're both on the "root" or "tip" side of their respective servos.


A VERY rough general idea below:

If you look down on the plane, and the servos look like this : =|| =||

Make them look like this: =|| ||=

Hope that makes sense.