cowriter

co·write

(kō-rīt′) or co-writetr.v. co·wrote (-rōt′), co·writ·ten (-rĭt′n), co·writ·ing, co·writes or co-wrote or co-writ·ten or co-writ·ting or co-writes To write jointly or in collaboration with another author.
co′writ·er n.

cowriter

(ˈkəʊˌraɪtə) n (Journalism & Publishing) a writer who works in collaboration with another writer