I have two refs by same authors but endnote puts them in alphabetical order inc. title of paper which is the reverse order of how they appear in the text. So at the mo. they are 2004b then 2004a. My supervisors bless their cotton socks say they must be 2004a 200b in text. Here's a silly example to show you what I mean:

In his work with leopards (Jones 2004b) he refers to his work with aardvarks (Jones 2004a)

Jones (2004a) Aardvark never hurt nobody
Jones (2004b) Leopards spotted in Tesco

has Jones (2004a) been referred somewhere else in your text before that citation?

no, I've only cited each once