Also, this is why MTH PS1 dual powered A units from the factory use the tether system and provide forward and reverse control to the DCRU in the rear unit to "slave" off the front unit. When you do this wiring, again, the DCRU board F-N-R logic function is disconnected, and now it's basically commanded F-N-R across this 3 wire tether system.
I would not recommend this for a novice- but it's just 3 wires, between the PS1 board in the front engine, to the DCRU board in the rear trailing engine