RE: Do I need to reprogram my 5645
If the servos are on opposite sides of the airplane I wonder why you are using two channels for the servo's. Trying to match the servos with mixing just makes it harder to match the travel of the servos. The programmer will help but it still will be hard.
I think it you want to hard connect two servos to a surface you will be better off not using digital servos. Their tighter feedback loop will make it harder to match them.