I remember reading about a homemade dual-RG6 speaker cable. The center conductor of one coax (let's call it 'A'), is paralleled with (joined at the ends) the shield of the other ('B'), and the center of 'B' is paralleled with the shield of 'A'.

I can't imagine any electrical benefit, except maybe for a high capacitance, but it otherwise seemed an excuse for using up a large surplus of coax. I would still run good ol' speaker wire, and only use a coax to feed an amplified speaker.