In the best case scenario, you finish all of your prerequisites prior to the second semester of junior year and take the April MCAT.
In the second-best case scenario, you finish all of your prerequisites during your senior year and take the August MCAT.
Or, if you are like me and finished a good part of your undergraduate education without planning for medical school, you can always return to college after graduation and finish prerequisites (if it is NOT easy to get into science courses at your school, do NOT graduate without finishing premed, even if it takes five years for you to get your diploma!)
I'm pretty sure that econ majors must take calculus, so you'll be able to take that when you have to.
Most econ majors at my school had to take three science electives. Here's where you fit in your two bios and a semester of chemistry.
All you are left with is a second semester of chemistry and then a year of physics and organic chemistry (a lot of people panick about having to take these). I took the latter two courses with biochem and calculus. There were more exams that semester and fewer papers than I was used to, but I still managed to do fine and have a social life.