rotten idea. get two sets of recs if you're applying to two kinds of grad school, IMO.
i know plenty of people who applied to med school and law school simultaneously, or MSWs and MAs, or whatever, but i would have someone write a law school rec and someone else write the other. if you want to use the same pople, either explain the situation and ask for two letters, OR make use of career services if possible and have someone make you a composite letter.