There are 3 kinds of master/slave systems on RC Txes that I know of:

1. Dry pots. The pots are read out through a wire each by the master
transmitter, plus GND. Slave has power off while doing this. This is what
Sean has indicated. Often has a 5 pin DIN (old system).

2. PWM rules. The slave has power off but receives power from the master
through the cable. Only the PWM part of the slave works, the Tx is off.
Often has 5 pin DIN.

3. uC to uC. The computers in the two Txs talk over the cable using a
proprietary protocol. Almost never uses DIN 5 pin, excepting some older
semi 'home-made' designs based on 8051 uCs.

hope this helps,

Peter