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BTW, is only one of the two exhaust pipes functional?
Also, it's a 335i!!
Just a guess... but it might be one of those systems where it's routed through one side when idling to keep it quiet and when WOT occurs a butterfly valve opens (or something to that effect) & routes exhaust through both side for maximum flow...
On my '02 Maxima it had double pipes on one side - during idle only one tip emitted exhaust - it was always dirtier than the other too.
Also - I'm not sure how an exhaust of that design could be that effective because it's a straight 6 engine. I do realize that the M3 does have two sets of double outlets.... for looks?