Carl,
everyone is correct, one winding with the two ends connected to your spark plugs (2). Once the points open the energy stored in the primary winding transfers to the secondary winding. Now it’s charged the energy is looking for the easiest path to ground. It could be either plug which has a little less resistance & goes to ground. The other plug “sees” less resistance ( thru the arc) & goes to ground too. Happens in a micro second.
Garry is correct! I used these dual fire coils in my Mercury motor because stock ones are not available . I grounded one of the secondary outputs as I’m firing just one plug. Again 50% less resistance gave me 50% more energy for a very hot spark.