Double majoring is a lot of work and will take longer than the traditional 4 years. I also expected to teach at the college level and after obtaining a Masters degree in history I ended up as an adjunct teaching 7 classes per semester and not making enough money to survive. I still expect to obtain my Ph. D in History later however since there are very few full-time professorships available (since most schools use adjuncts) I have transitioned into museum as it is a bit more steady. My advice would be to take your time and really decided what your options are. Explore the job market for the type of work you would like to do. I love teaching and will go back to traditional teaching at some point but museum work is also teaching the public.