Dyna coil failure.

Just had a 2 outlet 3ohm dyna coil fail ( cyl 1-4 ) on one of my Zeds which has now been fixed by replacing with new.
I replaced both coils while I was at it as they were both equally old (20 years)..
HOWEVER! only one outlet ( number 4 ) stopped firing which should be impossible and correct me if i'm wrong but both poles usually complete the secondary winding and if it fails both plugs should stop firing ?
The only explanation is that the Dynas have seperate internal grounds for each outlet or that the remaining functioning pole some how managed to find it's own ground by accident via an internal short .
Anyone know better ?

I do not know the answer, but I think this is a perfect excuse to cut the coil open and see what is in there.

One side could have found a high tension path to the core. Ohm tests wont necessarily show high tension paths because they can be open at low voltage.