On the early Wings where the coil fires on both wires at the same time, is it possible that just one of the wires can go weak when the coil heats up?

Unlikely one of the out put sockets would go weak but it is common for a plug wire to fail at lower revs and still work at high revs or when cold. Also poor assembley from the factory had #2 lead on mine only just making contact and this worked OK for the first 28 years only failing 3 months ago

I had it backfire on the left and I did everything possible to find out why but it still does it with open (aftermarket) pipes. Changed the wires,caps, plugs,... carbs synced, even installed a honed head on that side, new exhaust gaskets, went and switched the coils and it still does it.