Read this in an article about the c320.....same engine....

"These all-aluminum 90 degree V6 engines have an unusual valve arrangement, 2 intake and 1 exhaust, with 2 sparkplugs per cylinder, each with their own coil. The idea of the single exhaust valve is that the exhaust gas remains hotter, thereby improving emissions especially when the engine is still cold. A single exhaust valve also allows room for the 2 plugs which fire in sequence, first one then the other. This allows for,among other things, a much leaner mixture to be used for improved gasmileage and reduced emissions. After each firing cycle, the order in which the two plugs fire is reversed, allowing for a smoother engine with better control over engine noise."