To add to what Montague said...

On a properly configured buddy box setup, there should be absolutely NO change in the position of the control surfaces or throttles when you flip the trainer switch and back.

In other words both the master and slave must have exactly the same trim settings.

You should also test this at all stick extremes too...

e.g. move the right stick left on both the slave and the master, then flip the switch... there should be no change in the surfaces, which would reflect a difference between the two TX's...