I met a guy this weekend with a double-zippered coat that struck me as brilliant. It was a leather motorcyle jacket, with the traditional off-set side zipper, but where there normally is only one running up the side, there were two, about an inch and a half to two inches apart. I asked him about it, thinking maybe it was just a design feature -- the coat was one of those garments that are full of details that are ambiguous as to whether they're just design features or have some practical application -- but he told me no, it was so that when you wear a thick sweater, the coat can be a bit looser to accomidate the extra bulk.
Having sat and thought about this for a minute, I realize that despite the amount of time I've spent chatting about motorcycle jackets, between Den and his love of motorcycles and Carly and her job at Langlitz, I haven't ever really investigated the zippers on the jackets. Is the double zip a common thing?
Either way it seems like a great solution to the "buy it big enough for a sweater" dilemna that Scott Schumann writes about in The Sartorialist: "...put an end to a dreaded sentence heard in shopping malls all across America. 'buy it one size bigger so you can wear a sweater under it!' I loath that sentence."