Commas vs fullstops in in-text citations

This problem is doing my head in!

I have downloaded the psychological medicine output style, but i have to modify it to fit with current journal guidelines.  For this journal, two authors should be cited as (Addington & Addington, 1998).  But 3 or more authors should be cited as (Addington et al. 1998_).  So_ there should be a comma between author and year if only 2 authors, but there should be no comma between authors and year if there are 3 or more authors. Is it possible to do this in endnote? And if so, can anyone point me in the right direction. Ive been trying for 2 days to get this to work!