ORIGINAL: Dr1Driver
The black wire shorts the primary side of the coil to a known ground. A parallel connection will not kill the engine if only one switch is closed. You must use a series circuit. I use a sturdy SPST (single pole-single throw) toggle switch from Radio Shack. Simply run the black wire to one terminal and the other terminal to the engine block. You can use a micro switch activated by a servo for the electronic-operated killer.
Dr.1
It seems to me that if the two switches are wired in parallel, then either switch would short the black wire to ground. Where as, if they were wired in series, then both switches would have to be activated to short the black wire to ground.
Jim