From the book Aircraft Propulsion Powerplants, third edition of 1962 by Cargino and Karvinen.
'The Civil Aviation Regulations require that all engines be provided with dual ignition'.........and.......'In most cases, the two ignition systems are timed to fire both spark plugs simultaneously. However, due to variations in conbustion chamber design which affect the turbulence characteristics of the gases and the location of the spark plugs relative to the valves, it may be necessary to employ staggered instead of synchronised firing. With staggered timing the spark plug nearest the exhaust valve is fired from 4 to 10 degrees [crankshaft travel] before the other plug. This is necessary because the mixture around the exhaust valve is contaminated by residual exhaust gases and requires a longer time to burn'.