Hi,
I've read that coax cable is good for its self-shielding nature, i.e, if all of the coax center conductor current returns on the outer conductor of the cable, then there is no external magnetic field produced. However, this is not the case for 2 parallel wires, and 2 wires would produce external magnetic field.
I've seen explanation on the coax case which uses Ampere's law that said since the enclosed current of a contour outside the coax is zero, then there is not magnetic field. So, isn't this the same case for the 2 parallel wires when you have each wire carrying the same magnitude but opposite direction of current?