Bill,
I reread Jim's post I'm not sure which way he runs his set up but see my post #3 you can run your batteries in series if the GP are in series as well. To run the batteries in series (3V) hook up the positive lead to center pin on one glow plug and the neg to the other glow plug you are running a 3V system through one GP use the body of the engine as a conductor and out the other GP. You have the GP in series as well, it needs 3v to run correctly both get the same amount of current. Only down side if one GP opens (burns out) neither GP will light but that isn't all bad since you can't start the engine. It really works very well.
I plan to run my PBM twin using this same set up.
Nifty GP connector is to use 1/8" wheel collars, most are brass under the chrome plating drill another hole through the side of the wheel colar sand off the chrome and solder on the wire then use the jam scew to attach to the GP. easy cheap reliable as well.
Sparky