I am so sorry to hear about your disappointment from Northwestern. Had you spoken to the teacher you intended to study with if this was feasible at Northwestern? Many schools say they support double majors, but in actuality it is only if the music students are not instrumentalists or the academics does not include science labs.
But moving forward in your current situation. My understanding is that the majority of people that double major end up dropping one field or the other. That doesn't necessitate, however, that you be one of them. Have you seriously pursued, at an advanced level, both academics and science at the same time for most of high school? And if you haven't read it yet, you may want to check out violadad's great recommended discussion of who should be a double degree student. (He didn't actually write it, he was just the one that sent me to it first. ) You can find it on the Peabody website at
http://www.peabody.jhu.edu/doubledegree.
I would add to that rubric- Do you have your most fun doing either of these two subjects? Because as I see it, for the next 5 years, you will probably have little time for fun outside bio and music. You had best enjoy them a whole lot!