My experience with trying to split one plug wire to fire 2 plugs tells me it won't fire both plugs. One or the other but not both at the same time. I'm sure there is some law of physics, thermal dynamics or electrical engineering that boils it down to the path of least resistance but my experience with it is as follows.

A couple of decades ago I was riding across Canada on a CB750K Honda when it smoked an ignition coil outside of Dryden Ont. On a Sunday no less. The 750 used a wasted spark ignition with 2 coils and 4 plug wires to run 4 cylinders. I limped it into a wrecking yard just west of town and as luck would have it the owner was there so I bought a used delco coil a ballast resistor and 2 plug wires and assorted clamps and wires and proceeded to "fix" my Honda. I wired up the coil, spliced the plug wires together and it fired right up but only on 3 cylinders. I pulled one plug and then the other to find which one was missing figuring it was the splice but it would still fire 3 no matter what. Solution was another coil wired in parallel so each plug had its own coil and it ran like nails.

Kevin