Redressing a controversial subject...

Interesting idea. A conventional coil has about 100 millijoules of energy. Two coils in series have twice as much. Potentially a hotter spark. What that actually would improve is another question.

You could probably do it with one of those GM coil packs that they use to fire two plugs at once. On these both sides of the secondary are ungrounded. Put everything in series, use a points distributor and 24V supply and go to town. Have to figure out where to put the extra 12 battery.

Go for it, report back!