Women in Hats, though.
I sometimes thought that PTerry may have felt like writing a relatively non-nonsensical Doscworld-related story for a younger audience (which he did quite well, I like the Tiffany-books very much), and thus introduced Tiffany as the young heroine. I think that was also a reason for entering an area that wasn't really mentioned before in the Discworld books, to give it all an independant stance.
That might enable him to keep the two witches apart, despite Granny's role in Tiffany's life.