This project solved a problem involving a momentary contact closure x 2 involving 2 wires.  Where 4 wires of the proper wire guage were needed.

What  if you needed to remotely maintain a contact closure and needed the status of that contact closure. Then two computer chips communicating using tones on those 2 wires could talk back and forth. 

A protocol could be devised where there is a normal banter back and forth in half duplex ( each taking turns talking ). Then the number of contacts could possibly be in multiples of 8, and each contact can be ON, OFF, PULSED, or TIMED as needed. With a status report on each and every one.