How do you cite two or more references within the same parentheses?
Order the citations of two or more works by different authors within the same parentheses alphabetically in the same order in which they appear in the reference list (including citations that would otherwise shorten to et al.). Separate the citations with semicolons.
Several studies (Miller, 1999; Shafranske & Mahoney, 1998)
Arrange two or more works by the same authors (in the same order) by year of publication. Place in-press citations last. Give the authors' surnames once; for each subsequent work, give only the date.
Past research (Gogel, 1990, 2006, in press)
Identify works by the same author (or by the same two or more authors in the same order) with the same publication date by the suffixes a, b, c, and so forth, after the year; repeat the year. The suffixes are assigned in the reference list, where these kinds of references are ordered alphabetically by title (of the article, chapter, or complete work).
Several studies (Derryberry & Reed, 2005a, 2005b, in press-a; Rothbart, 2003a, 2003b)
Exception: You may separate a major citation from other citations within parentheses by inserting a phrase such as see also, before the first of the remaining citations, which should be in alphabetical order.
(Minor, 2001; see also Adams, 1999; Storandt, 2007)
(adapted from the sixth edition of the APA Publication Manual, © 2010)