According to a top guy at Cosworth .... if the spark plug is in the correct place to start with then not much point in twin plugging.
If not then twin plugging will be of benefit. They had done much experimentation!
Was talking to the guy at the NEC motorcycle show a few years ago .... in the BMW hospitality suit I might add
The thing with airhead heads, is that the plug is very much not in the right place. It's on one side of a hemi head - long flame path.