As a matter of fact, I know some London Tube lines used dual-third (4th?) rail systems, and I think a few systems used dual-overhead wires as well. It eliminated the issue of galvanic corrosion in water pipes, from current leakage along the rails. No reason a dual-line system couldn't be used in a modern trolley pole-equipped system, and even a pantograph-equipped car could use dual overhead lines, with either a large neutral zone at a junction or some sort of current-switching system as cars passed.
The dual third-rail system is still used today on the London underground. Condiut current collection in London and Washington was like this too and how unfortunate is it that there is not one tramway still uning the system. Would Washington have retained conduit curret collection had it survived, or would it have been replaced by something else, such as the Bordeaux ground level power supply?
Pantograph equipped rail-vehicles could use dual overhead lines, but the air-gap (between the wires) would need to be wider. This was used on some polyphase electric railways, in this case there were two wires for two of the phases and the rails for the third.
EDIT:A minor hazard associated with rail return is that if sand builds up on the rails the rail vehicle can become insulated, causing the whole body to become live at the line voltage. I believe that on the most modern trams and LRVs, the procedure would be to lower the pantograph before opening the doors.