Dirty Bird,
Yes, the plugs are in series with the battery and switch but at the point you connect the two tips of the plugs to the common positive lead and then the common negative lead to the engine block puts the plugs in parallel with each other thus providing the same voltage drop across both resistances (coils) assuming they are of nearly equal resistance. If you are using two 1200 mah sub c cells in series with each other then E=2.4v and you are over driving the plugs as I am sure with your background you already know.
To clarify my earlier post, what I tried to say is that to put two glow plugs in series with each other would require an insulated threaded end of one plug connected electrically to the tip of the other plug thus making the voltage drop =2.4v. That scenario is a little hard to achieve as I dont know of an engine manufacturer who provides an insulated glow plug socket for thier engines.


blabree