I think it's possible, but I think you should also consider that you could do the type of research you want to do in a graduate program outside of math. I am doing mathematical/computational biology work for my PhD at the University of Wisconsin. My advisor's main appointment is in the math dept, and I have an office in the math dept, but my graduate program is biophysics. My undergrad degree was in math and biochem.
The reason I'm encouraging you to consider doing a PhD outside of math is this: the mathematics graduate program has a ton of required coursework, plus very intense qualifying exams, plus heavy teaching requirements, etc. I could be wrong, but I suspect these types of requirements are not limited to UW's math program. The math graduate students are very busy with all these obligations, and they barely have time to do research. That is why you hear about math graduate students taking 6-7-8+ years to finish. Biophysics, on the other hand, has very few requirements. I only have 3 required classes, which I will have out of the way this semester (first semester of grad school), so I am free to take whatever I want (within reason). I'm looking at taking some classes in math, stats, and computer science. If you are driven to get both the MD and PhD, I highly recommend at least trying for MD/PhD programs rather than doing them separately. After doing a math PhD, you may lose steam and not want to continue on to the MD...which raises another question, is the MD really necessary for you? How do you see yourself using it?
Anyway, I hope this helped a little. Feel free to message me or reply here -- I'm happy to give whatever guidance I can, especially for a mathematically oriented comrade!