Can I use microphone leads to carry Y/C video signals?

I work in the live music industry and frequently need to get video signals around and about the place. Usually I run long legths of 75ohm coax and send composite video down them. However since most of my equipment will output Y/C I have been considering using this format - clearly the signal quality is better, but I have to run twice as much cable.
Just about every other signal in my industry can be run down a 3pin XLR lead, consequently every venue is always full of them. I was wondering if I sent Y/C using a 600ohm 3-way mic lead would I get a better picture than by sending a single composite signal down one bit of 75ohm coax?

Also if I were to do this, should I connect the two grounds together or leave one of them floating?

And, if I were to send such a signal through a multicore cable carrying many other signals, would it be likely to interfere with them?

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