[Bob W.]

On a single plug engine, the ignition location is off to one side of the combustion chamber. When ignition occures the flame front has to travel across the chamber to the other side for complete combustion. This takes a specific amount of time and requires the ignition to occure sooner as the engine turns faster so that peak pressures occure near top dead center.
With a twin plug system and two flame fronts on opposite sides of the chamber, the amount of time for complete combustion is shorter. With a shorter combustion time, less ignition advance is required. With less ignition advance you will have less detonation problems.(you can run more boost or compression without detonation with reduced ignition advance.)