Another linguistic paradox, just like what is right: pop or soda!!
Supplementary apps and secondaries are virtually the same thing, except it depends one what the school want to call it. The term secondary has been handed down to the predents from the Pre-med/dent confused students.
Doing what Matt suggested is the best and easiest course of action.
99% of schools have some sort of supplemental/secondary form. For some schools it requires the applicant to write 10 essays, and for other schools it is merely a form to determine where the applicant has legal residency (you put your name and address). So, they vary in form and function.