What happens you get EMI interference during the spark and it can travel to the core of the other wire. We had to be super carefull on aircraft with certain wires. Had a good one with a old E-4 compass system on a c-130 took forever to figure out only during a right turn the compass went whacky, turned out a wire from the wing tip flux valve (senses the earths magnetic field) was loose and would hang near a ac generator wire. Just found it by sheer luck.