ORIGINAL: RCIGN1
I just got some really strong magnets..They will trigger the hall sensor from about 1/8"..Any ignition that will run an engine will easily jump a 1/8 gap--mine will go at least 5/8...You can't isolate the spark plugs from the engine, they're connected by virtue of being threaded in, so both plugs are automatically grounded...Spark goes to both plugs, jumps the gap on which ever is smaller....May be one, then the other...2 coils connected in parallel will easily spark from ONE circuit and ONE sensor...
The plugs are screwed into the engine but the engine does not have to be connected to the circuit ground. As TKG says the secondary of the coil could be connected to the two spark plug leads, one end to one spark plug and the other end to the other spark plug. It just might be a better way to do it.