A question about MLA format?

It says in my handbook that when you have 2 authors with the same last name, put each author's first initial, or the whole first name if the authors' initials are the same. So if I have a book with two authors, Dane and Mary Coolidge, is my citation (D.M. Coolidge 19) or (D.M. 19). Or is it (Coolidge and Coolidge 19)?