The way it usually works, Patterson will write a detailed outline — sometimes as long as 50 pages, triple-spaced — and one of his co-authors will draft the chapters for him to read, revise and, when necessary, rewrite. When he’s first starting to work with a new collaborator, a book will typically require numerous drafts. Over time, the process invariably becomes more efficient. Patterson pays his co-authors out of his own pocket. On the adult side, his collaborators work directly and exclusively with Patterson. On the Y.A. side, they sometimes work with Patterson’s young-adult editor, who decides when pages are ready to be passed along to Patterson.

It appears that the idea for the book originates with Patterson, but his "co-"author actually writes it. I tend to agree that the co-author should be listed as the primary author and Patterson as a contributor. However I would assume that Patterson is the individual that negotiates the contracts with the publishers over how much he gets for each book "he" writes and then pays his co-author.