I think combustion theory is a very complicated subject. In aircraft engines the twin plugs are a safety fetcher, however, the added plug does add power. You can see that during runup. One component of the combustion cycle is called the flame front and begins the moment the plug fires. It runs across the combustion chamber at a rapid pace causing a spike in cylinder pressure as it travels. In some cases the pressure build up is so hi that some unburnt AF mixture detonated. What we call spark knock. The use of twing plugs can eliminate this with well placed plugs by shortening the flame front distance. Thus the logical place to put it is at the top and center of the chamber or over the piston and transfer area.
Automotive engineere have been moving the plug around for years and still arn't quite sure where to put it. The advent of the power tip plug is another improvement in the combustion process. by it's very nature it's pretty self cleaning. Unfortunately they can't be used in stock heads. I'm making a fixture to angle then away from the exhaust valve. Thes can improve the economy of your engine by burning a leaner mixture. I'm sure this subject will be tossed around abit, which is good for the advancement of a better runnng flathead.