As I have posted before, there are too many dental prerequisites to forego any number of years in the dental school curriculum. I would say that the freshman year is composed of 50% basic sciences (where having an MD would allow you to bypass) and 50% dental sciences (where having an MD means squat). Then in the 2nd year, 25% is basic sciences and 75% is dental sciences. From 3rd year on, it's all dentistry. Even the basic sciences is geared toward teeth and gums (Oral and Microanatomy, dental neuoscience, etc., etc.). Those courses focus much more time on the development of teeth than med school microanatomy courses...and if you don't have that background, you will not understand the basic framework of dentistry.
Your reasons for wanting to get both degrees is warranted, but to seek a shortened time frame for getting both degrees is nearly impossible. It seems as if the real problem with the dentally-underserved areas is the lack of dentists in the area, not the lack of physicians. My suggestion is to go straight into dental school, get yourself a DDS at LSUSD, and then get on out to those rural areas and get busy.