Blowing Coils

[Nourish]

I originally used the standard 1983 electronic ignition (BMW 12.14-1 244 478) with two PVL 356 1 00 coils (the ones that Motobins sell)
Well back in June I got stranded -no sparks.
As I thought if one coil had failed at least the other would fire I replaced the ignition unit with a Power Train 8100 052 listed for a Volvo and supposedly matched the BMW but no it was one of the coils had failed which I replaced with a cheap copy off E bay.
And now I’m here where I am today with the Power Train ignition module on the bike,wiring up the old red(orange) BMW twin output coil back on I get a spark - so it is the coils failing.

The primary on the cheap copy is 1.0 on the 200 scale but I can’t test the secondary resistance as the terminals pulled out with the HT leads!
The blown PVL coil has a primary resistance of 1.1 on the 200 scale, the secondary measures 8.65 on the 20K scale
The previous U/S PVL coil just for reference measures primary 0.9 and nothing on the secondary

The spec from the PVL site is:-

High voltage supply: > 21 kV
Max. spark energy: 30 mJ
Primary resistance: 0.6 ohm
Primary inductance: 1.65 mH
Secondary resistance: 8.7 kOhm
Secondary inductance: 9.2 H

PS, As the 'Blown' coil looks to be within spec I just wired it back up and it still works!

[Nourish]

I thought the trouble with ensuring that each twin output coil feeds both cylinders on a twin plugged engine is that you never know if a coil should fail.
But why would there be no sparks if just one of coil fails?
Now I can understand that if one coil fails then the other may well be under some strain.