You mentioned "12 plug distributors" so you've probably seen these, http://www.raceservice4u.com/Twin plug.htm Just buy the cap and rotor and make an adapter ring so it would fit your distributor.
You are aware that unless you have a true hemispherical combustion chamber and piston top, like the old Harley engines used, or a cylinder of ~5" diameter, there is no real advantage to dual plugs. With a flat top piston and small, ~4" diameter, any difference in time of ignition start will probably increase the chance of pinging/detonation from flame fronts colliding. For some reason you don't want to share the make of engine your working on, so there is a lot of guessing going on in the answers. Better combustion chamber shape works much better than dual plugs for most engines.
You are aware that unless you have a true hemispherical combustion chamber and piston top, like the old Harley engines used, or a cylinder of ~5" diameter, there is no real advantage to dual plugs. With a flat top piston and small, ~4" diameter, any difference in time of ignition start will probably increase the chance of pinging/detonation from flame fronts colliding. For some reason you don't want to share the make of engine your working on, so there is a lot of guessing going on in the answers. Better combustion chamber shape works much better than dual plugs for most engines.