I was thinking of a series circuit. Hot side of the coil to the positive side of the kill switch in the cab. Negative side wire from that switch to the back switch hot side, and negative side of the back switch to motor for ground. This way if either switch is switched off it kills the mag. Any thoughts?
If you do the series as described I don't believe it would work as intended. With what you described, if you flip the cab switch to kill the engine all you'd be doing is sending voltage to the rear switch and with it still "open" the engine would continue to run.
I'd say the two switches should be "parallel", with the coil wire going to the "hot" side on both switches with the negative sides going to ground. With that, if either switch is flipped to kill the engine, the coil's current would go to ground and kill the engine.
I'll admit to having no experience with a mag in a drag car, but I believe I understand the kill switch process. If I'm wrong, someone PLEASE correct me and I'll crawl back under my rock.
Best wishes.
Lynn