If you use several sources in the same part of the text, these can be placed in the same paranthesis.
The order of the authors is as follows:
Different authors: The sources must be listed in alphabetical order, separated by a semicolon.
Example: (Repstad, 2007; Riis, 1996)
Same author: The sources are listed in chronological order, oldest to newest, with comma between the years.
Example: (Wassmo, 2003, 2006)
Insert the first reference the usual way, using Insert Citation. Then do it once more, with the other source you want included. EndNote will then put the two references in the same parenthesis.
Go to Add/Edit Citation as usual, and search for the first reference you want to insert. Then search for the second reference. They will then be placed next to each other in the search field. Press Enter, and Zotero will put them in the same parenthesis.