When using Tx's with manual trim pots frequently the pots (trim levers) on the two Tx's won,t match up well. Just setting the trims of both radios in the same place does not always get to close. Its handy on a first flight to put some one on the student box that knows how to fly and have them trim the airplane on the student (slave) box while you hold the trigger.

Another way to do this is with the airplane on the table using the master keep triggering the buddy box and look at each control surface one at a time. Adjust the slave box trims so that the surface does not move at all when the master triggers the buddy box.
John