They are actually pen names used by multiple authors for the Nancy Drew and Hardy Boys series, respectively.
* Carolyn Keene is a pseudonym used by a team of authors, including Mildred Wirt Benson, Walter Karig, and Leslie McFarlane, among others.
* Franklin W. Dixon is another pseudonym used by a team of authors, including Leslie McFarlane, Will Cooley, and George Ferris, among others.
The use of these pseudonyms was a common practice in children's literature to create a consistent brand identity and to keep the focus on the characters rather than the authors.