My understanding is that the second plug is used to accelerate the burning of the compressed air/fuel mixture. In the twinspark, the plugs are offset from the center of the combustion chamber. Firing a single plug would result in a longer time for the flame front to reach the far end of the combustion chamber, also requiring more ignition advance. Firing two plugs gets two flame fronts going to burn the full mixture more quickly. I don't see a benefit in delaying the second ignition event in our hemi heads.
Jim does that fit with your understanding? By the way, I love your 4 cylinder book.
'71 Alfa GTV
'88 Porsche 911
'02 Audi S6