One coil will not do the job if its not actually built to fire two plugs at once. If you’ve got two high tension leads off it, that’s fine, but if you’ve only got one high tension lead from the coil, my experience is that you’re fighting an uphill battle to get it to fire both plugs at once.

Most Model T buzz coils are built to fire one plug. If you try to wire it up with two plug wires, the path of least resistance rule can come into play, and you will end up with spark at one plug or the other, but not likely both at the same time.

FWIW – in 1939, Elmer Kiekhaefer (you may know him better as “Carl”) solved some of the Thor-Sea King ignition problems by installing twin-cylinder magneto coils in the Thor singles, and rewiring them to deliver the full voltage to one plug. I’m not sure why I brought that up…. probably because I’m an idiot…….

Theoretically – You could use an Elto Light Twin or Speedster coil and it should accomplish what you’re after. I don’t have a battery-ignition Koban, the one Koban I do have uses a flywheel magneto. Only twin-cylinder battery-ignition motor I have is some stupid thing called an Aerothrust, which currently is wired to use two buzz coils.

Hope this helps eh

Best,
PM T2