Teaching two different subjects? I am a 2nd year Math Major, and I am aiming to recieve a PhD in Mathematics. I love mathematics, but I also have a longing for African American studies. I want to be a college professor, so I was wondering is there any feasible way I would be able to teach both subjects? If so, would it involve me double majoring now or am I allowed to start two PhD tracks in different fields. Thank you.
Joint appointments across different departments at a university are not that unusual, and at many schools are in fact quite common. However, such cross-appointments tend to be within "branches": a scientist might be cross-appointed in multiple science and engineering departments, but not likely in both a science department and a humanities department. 

So it's unlikely that you'd get an appointment in both mathematics and African-American studies.

However, that doesn't mean that you wouldn't be able to teach both those subjects. You might be able to convince a university to let you offer an elective seminar course in addition to your "primary" teaching in mathematics. However, this is by no means guaranteed.