When it stayed that way, you must have been using a style that didn’t alphabetize the citations. You can’t ask endnote to do that on one citation and not the rest, as far as I know. The best option is to hide the part of the citation and type what you want to appear as a prefix so it looks something like this, but you will need to make sure the year with the b is correct {Elmore, 1996b, p. 1`; see also \Berends, 2001 #1856;Cuban, 2001 #2085;Tyack, 1992 #1548; #1859}.
I believe a request of this kind has been made before, but it is worth adding to the suggestions forum. I suspect it isn’t a trivial programming request though.