Hold on there. It's not that simple to be both an M.D. and a D.O.
Firstly, those DOs who graduate in an AOA-accredited school who then enroll in that MD diploma mill in Antigua cannot refer to themselves as MD/DOs. This comes down to an issue of licensure. Those MDs from Antigua do not qualify for licensure, and thus the physician in practice is licensed as a DO and cannot, by any means, be an MD. This would be fraudulent and illegal, as in punishiable by fine and revokation of licensure to practice medicine.
Secondly, graduates of the NYCOM APEP program are DOs and NOT MDs. I understand the fact that they were MDs in various foreign nations, but if they enroll in the NYCOM APEP program for the purposes of practicing medicine in the United States, they will graduate with a DO and cannot legally refer to themselves as MD/DO. This goes for DOs who went to Ross or St. George's, graduated with an MD, but never obtained licensure as an MD.
There have been numerous cases in New York State where AOA-trained DOs who obtained MDs from legitimate and non-legitimate medical schools overseas have been fined and had their licenses suspended for fraud and misrepresentation. The Office of Professional Misconduct (OPMC) has logged several cases of this, and I have personally read the brief for case against a DO in New York State who obtained an MD from an off-shore diploma mill. He was subsequently found guilty, had his license suspended for a long time, placed on probation, and fined heavily.
The ONLY way DO/MDs can exist legally is, as was mentioned, the California merger of the 1960s (in which the MD would be from and only from the "California College of Medicine") OR if there is a DO or MD graduate who decided to go through the requirements of the other program to obtain licensure.