Campy is correct except that the Futaba cord works OK in either direction. The Hitec cord may, or may not, have a diode in it. If it does, it will be marked as to which end is the master & vice-versa.

The student (slave) box must remain off at all times -- it is a good idea to remove the Tx crystal from the slave box to prevent an Oh ***** if it is accidentally switched on.