Then, one at a time, simultaneously test each control surface and direction by giving "full" input in both directions. I,e. move give full right aileron on each box at the same time, then flip the buddy switch back and forth. Again, there should be no change in the plane when you flip the buddy switch. Repeat for all surfaces in all directions (full left aileron, full up and down elevator, etc).
Matching both the master and slave boxes at full surface deflection is not important and can't always be easily done. If the slave box is a non-computer radio there is no way to adjust the end points and adjusting the master isn't always feasible. Just make sure the slave box doesn't produce more surface deflection than the master as this will make the sticks touchy for the student. It is important to make sure the surfaces are matched on both boxes at neutral as this is where they will be when control is shifted to the student. The throttle positions should be set on
both boxes at a comfortable speed for the student and so the speed does not change when control is given to the student.