Well technically it is, but only because it was more advantageous all-around to release the movies separately. I mean they were made at the same time, and the story carries through each film; unlike, say, Shrek 2 or Toy Story 2 or Miss Congeniality 2 or whatever which were simply made after the first one succeeded. (some better than others, of course...)
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Lather Rinse and Obey!