Here is an essay you might find useful:
http://www.peabody.jhu.edu/conservatory/admissions/tips/doubledegree.html
I know a few students who have double majored (BA program) with music and, say, French or literature. Combining music with STEM would be tough but people do do it.
Double degree programs are 5 years and can be BA/BM or BA/MM. The BA in the latter is of course not in music, and in the BA/MM program it can also be in something other than music.