In my hands, Endnote doesn’t allow me to use the Author (Year) format for multiple citations, mostly because it doesn’t work? You can go into the ‘more’ option and choose it for each, but it still doesn’t work properly. I can’t even get what you have to work, without then hiding an author as well.
The only way to achieve this, is old school. Leave both as (Author, year) and hide both authors, and type in the author ahead of the parenthetical years.