If a publication has two authors, the surnames of both are mentioned. In subject-oriented referencing authors are are separated with an ampersand (&). In author-orinted citing their names are separated with the word "and".
If the source has three or more authors, only the first author’s surname is mentioned, followed by the abbreviation “et al.”.
To avoid ambiguity when referring to two or more publications by the same authors, mention a sufficient number of authors in the in-text reference so as to distinguish the references from each other.