I believe what you need to modify is in the Author Lists section under Citations for that output style. You specifically would want to change the Abbreviated Author List - First Appearance and Subsequent settings so that the second bullet is "If 3 or more authors, list the first 2 author(s).
Have you checked the citation’s “Author Lists” setting of your output style? [Go to the toolbar, select EDIT, OUTPUT STYLES, EDIT name of style. Then in the output style locate “Author Lists” under Citations - see attached image].
The Author Lists setting determines how the authors will be listed in the in-text citation. Your style may not have accounted for more than 1 author so it would just be a matter of correcting the options. Once completed, close the style window. You will be prompted to save the changes to a file with the word “Copy” added (i.e., APA 6th Copy). You will need then to change the output style setting in both EndNote and Word to use the file copy instead of the original output style. Otherwise changes to the Author Lists settings will not be displayed.
Also note that the bibliography has its own Author Lists setting also. So adjustments, if needed, will need to be done separately.
Check your My Documents folder to see if there’s a sub-folder named “EndNote” - if so, the modified output style may have been saved to to this location.
If not, use your computer’s search capability to search through the hard drive to identify where the output style file is located.