This is what lost me after reading Kommando's comment from 2013.
How does an engine ground function for one and not the other?
On dual coils one coil is wired opposite polarity to the other internally so ignition spark current stays within the coil/head loop and so both always fire if the coil is good.
On twin coils you have a full loop including the battery.
With wasted spark you have double the ignition current than with points, on my bike the engine earth wire was failing and so when 2 coils fired there was not enough capacity for 2 sparks but enough for 1 spark so only one plug sparked. When I added the dedicated earth the capacity for 2 sparks was restored.
What gave me the idea was when I took the HT off the 'Good' coil and started the bike on only one cylinder the 'Bad' coil sparked well with none of the previous backfiring or no spark.
New N7Y's gapped .028.
Swapped HT leads. No change.
Coils are 2 6V PVL's in series. Swapped in spares. No change.
Checked/snugged trigger wire terminal block screws. No change.
I did all that and changed EI completely, all made no change until I left the HT lead off the good coil and then I realised there was an issue further down the path and added a new earth wire, instant restoration of 2 sparks.