But I expect the answer is going to be – that there is no way for the rt click to know which author to hide. 
- and yep, there it is in your reply below (or above depending on how you view these).
yes, finally found it, but not very intuitive - very many clicks.
Honestly, how often do you think there is going to be a grouped reference with the first author having the same last name and it not BE the same person? – I know, I know. It only has to happen once and the user will be irate that it isn’t handled correctly.
But the obvious correct behavior would be for Endnote to first check if the primary-first author is the same, and if not impose the disambiguaton steps (2008a, 2008b and then adding initials for your example), and then to apply the omit to the identical first author group. That is how they will end up in a publication citation for most journals I have come across.