Many moons ago, someone had the idea of having a buddy-box system where the student's tx had servos installed that would move the control levers. The idea is to give tactile feedback via the buddy-cord to the student as the instructor demonstrates how to move the sticks.
This was called a "shadow box".

They assembled two types of shadow boxes. One was a mechanical contraption with rods and cables from an active tx to dummy box. The dummy box had all electronics removed and only the sticks and gimbles that remaind attached. Again for tactile feedback purposes only.

The other shadow box did not have a hard connection to the two tx's but did have a flight pack installed in the student's inactive tx. That why although the student didn't have control of the aircraft in flight, she or he could feel the correct stick movements as the instructor flew the aircraft. It was reportedly a resounding success for the purpose of training.

Take care,
CCR
http://www.kites.org/rc_instructors