Canaan--Some Boucher banjos had double heads, but I've never seen any others. I would say there is no advantage, since they certainly didn't catch on. A couple things, however. Drums had two heads and there were all sorts of experiments with banjos that didn't catch on. For example, I have an 1885 Charles Dobson banjo with the tone ring mounted outside the rim. It works fine, but doesn't seem to add anything to the sound, so it wasn't replicated as far as I know.