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When second cat is heated up, it cleans up well enough (many had passed MOT checks if their local rules aren't anal about mods like in CA), even if it's only one. Unfortunately newest smog/exhaust standarts measure also from cold starts, hence extra 1st cat is placed much closer to cylinders, where it happens to be robbing lot of performance, especially for NA. But it heats up to working temps there quicker. From all of this I wouldn't say, that 1st cat does most of work .. it just starts working sooner.
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