The secondary winding has two ends, one to each spark plug. To test use an ohm meter & connect each end to the spark plug leads & read resistance. When a “dual fire” coil discharges the end that has the least resistance fires first, then the coil “sees” the path to ground thru the spark plug there is enough electrical energy in the windings to “fire” the second spark plug. NEVER put the two spark plug leads together as the energy has no where to go & might arc to ground thru the windings. Once you make the electrical energy it HAS to go somewhere & will blow a hole in the windings to the mag plate, then the coil has very high resistance from the carbon arc or it’s completely open, either one is coil failure.
If you ever work with relays in a D.C. circuit, there must be a doide across the two winding terminals as once the power is removed the winding now has an electrical charge & must go somewhere. That is why a diode is placed across the relay winding to discharge the stored energy. The diode is a one way electrical valve.
Also must be put in the correct polarty.
