It is a "waste spark" system. Both spark plugs fire at the same time... one on it's cylinder's compression stroke and the other on it's cylinder's exhaust stroke (where the spark is "wasted").
The secondary winding is NOT grounded at one end or connected to the primary (as it is in typical ignition coil).
A lead from each end of the primary winding goes to to a spark plug. One fires with a positive-going pulse and the other with a negative-going pulse.
MANY automotive engine from the 80's up use the same type of system, having half as many coils as there are cylinders.