In response to twinman:

I'm not sure which setup is best - series or parallel.

In the parallel setup, it is true that one plug could "hog" all the juice and the other plug(s) not get hot at all. However, all plugs will always have voltage applied and a dead plug will not kill the hole system.

In the series setup, all plugs will see the same current flow and will all get "fire". However, if one plug fails, the other plugs will not get any juice and the system is essentially dead.

In the case of a twin, either of these situaltions is not good while in the air. Failure of one plug means loss of one engine - bad jue jue [:-]