Complex sentence: By that Christmas, one million copies of the album had been sold; by the following year, it had sold an astonishing 7.5 million copies--unprecedented for any album, let alone a comedy album.
Split into simpler sentences: By that Christmas, one million copies of the album had been sold. By the following year, it had sold an astonishing 7.5 million copies--unprecedented for any album, let alone a comedy album.