[sokoloff]

Minor point: it's two magnetos, each magneto driving a separate ignition harness and spark plug in each cylinder.

Each system is tested in the pre-flight by grounding out (shutting down) the left magneto and making sure the right magneto can run the engine and all those plugs are firing, then repeating the test with the right magneto grounded (shut down) and making sure the left mag, harness, and plugs are capable of running the engine.