I've never met or heard of anyone that is a MD or a DO and went back to school to earn a DDS or a DMD degree for the sake of getting a dual degree. I guess why would they (MDs and DOs)unless they wanted to be an Oral & Maxillofacial Surgeon?!
From what I know of 6-year OMS residencies that award a MD degree at the end, the program participant will have to complete the two year clinical rotation medical school curriculi (year 3 and 4 of med. school). A sample of an 6-year OMS residency program:
Year 1 - OMS residency
Year 2 - OMS residency (include anesthesiology, etc.)
Year 3 - 3rd year medical school
Year 4 - 4th year medical school
Year 5 - Various Surgeries rotation
Year 6 - General Surgery residency
Now lets say that we have a MD or a DO that wanted to be an Oral Surgeon, I'd assume that (s)he would have to complete at least a year or two of dental school curriculum to learn about dental anatomy, morphology, periodontium, and occlusion. Since general 3rd and 4th year of dental school curriculi includes rotations in dental specialties (Endo, Ortho, etc.) and comprehensive patient care, I would think that our MD/DO OMS wannabe does not have to do the"traditional" 3rd and 4th year dental curriculi!
I don't know, this is a very interesting circumstance and situation. I wonder what would dental schools do to tailor a MD or DO's interest to pursue an OMS certification?
POINT OF CLARIFICATION: Oral & Maxillofacial Surgery is THE ONLY way for a DDS/DMD to earn a MD degree. I do not know of any allopathic medical schools in the United States that will admit DDS/DMD applicants and bring them into their curriculi as MS-III to earn a MD degree! If a DDS/DMD does not want to do Oral Surgery and WANTS a MD degree, (s)he MUST apply to medical school just like everyone else including taking the MCAT.