EN putting commas between authors in author (year) in-text citations

Not sure how you are intending to cite these in text.  I suspect the Author settings are incorrect/incomplete as it is rarely used in a numbered output style.  If you want authorship to be reflected in some way, I would edit the output style to include the

, et al

for both of the 3 or more records as I assume you plan to use the authors’ names as a noun in the sentence structure and anything else, as you note,  would look odd.  

“save as” to a new style name and then change your document to use that newly named style (which you may have to show all styles, to find the first time).  

I don’t see this construct used in a quick view of a few published papers, and I don’t think they adequately address it in the author guidelines here: http://royalsocietypublishing.org/instructions-authors#question3  Mind you - they do direct you to the Endnote syles.  You might  ask the editor to have Clarivate correct the output style to reflect their preferences.