The two plugs actually fire independently and may not fire at the same time depending on engine operating conditions.

The 3V arrangement precludes a centrally located plug, which is best for shortest flame travel and combustion time, which is best for both detonation control and thermal efficiency, so twin plugs get around this by creating two flame fronts.

It's not really a new idea; 3V twin plug engines have come and gone over the decades.

Reciprocating aircraft engines also have twin plugs fired by two completely independent magnetos, but in the case of aircraft engines it's a safety issue. If one ignition system fails, the second will allow the engine to continue operating, though at reduced power since the ignition systems are timed for best power when both are firing. If one quits the timing with just one plug would have to be advanced for peak power.

During engine runup prior to takeoff a pilot should always check both magnetos. The switch has postions for 1, 2, and "both". When switching from "both" to 1 or 2 there should be a specified RPM drop. If not, that mag is dead and you don't take off.

Duke