There are engines out there with 2 glow plugs in the head.

However, I'm not at all sure a bigger plug would make running inverted easier. It might, but it might also make it worse. A bigger plug would mean a bigger area for fuel and oil to pool.

And a bigger coil isn't necessarily a great thing. Glow plugs work by creating a catalytic reaction between the platnum in the plug and the methonal in the fuel.

If you can afford some custom machine work, you could probibly get a duel plug head made for your engine and see what it does. At least with two normal size plugs, you create two ignition sources, which, in theory, leads to a more even and complete fuel burning, at least that's how it was explained to me. I've no idea if it really works well or not.

Oh, and fwiw, not all plugs are created equal. You can get heavy duty plugs that do have much stronger elements for high compression engines and such.