According to APA style regarding parenthetical citation, you will need to use the surnames of both authors separated by the symbol (&) and followed by the year for example (Robinson & Pearl, 2004) in case you want to refer to one written book by both authors. On the other hand, if you are referring to two different books and considering they share the same surname, you will have to include the initials of the authors, the year of each book and separate both authors using a semicolon. In this case, you will have to use the following format ( E. Barret Browning, 1990; R. Browning,1981)