Deferring is usually school specific, so you'll have to ask them. Then from there, they might ask why you want to defer (sometimes only granting it under special circumstances). If you're not honest here, it can really hurt you.
If they let you defer no-questions-asked, then I suppose what you are proposing is possible, but it is definitely a sticky situation.
I would like to echo some of the other responders:
Admission to a medical school is a valuable thing and by following the plan you proposed you may be gambling with your acceptance. I agree that MD/PhD programs are typically more competitive. However, if it is your passion to earn the dual degrees, then that may be a good reason to pursue alternate schools. First, I would recommend checking with the school you were accepted to, to see if they ever allow current MD students into the dual degree program (many schools do if they have the funds).
Final advice: tread carefully
-senior medical student / admissions committee interviewer