Multiplexing signals of two wires of a BNC cable and transmitting over a single wire
The two wires of BNC are not different signals, they are "signal" and "shield". The shield is connected to "ground", and serves to keep the signal clean and inside the cable.

For short distances, you can tie the shield to mains earth at both ends and use your single bare cable. This may introduce interference on nearby TVs.

How long is a short distance? I'd expect 10cm to work, 1m to work with some "snow" interference, and 10m not to work very well.