The APA 6th style has the following setting which is causing the behavior:
You can edit the style to change the behavior but since you have more than one reference from Casano in 2016 this setting is why it shows another author in the citation in the body of your document.
If you uncheck the setting in the style and use the modified version of APA 6th in your document then it would do Casano et al., 2016a and Casano et al., 2016b.
In EndNote you can go to Edit > Output Styles > Open Style Manager > locate the style you are using > Click Edit > go to Ambiguous Citations under Citations > here uncheck the box Add more authors until the citation is unique > go To File > Save As > (EndNote will not allow you to change the existing style in EndNote but will always apply the change to a copy of the style) Click Save to save the changes to a copy of the style > close the style window.
To get the change to show up in a Word document > Go to Tools > EndNote > Format Bibliography > Click Browse > locate the copy of the style on the list > Select the Copy > Click OK.
Please let me know if you have any questions or if this does not resolve the issue.