For many (maybe all) of the dds/mba programs you apply while you're in dental school. So maybe get through your first year in the dds program and get a feel for dentistry and how you want your career to unfold. Then, if you are still interested in the dual degree use that first summer off to study and ace the GRE.
This degree has always interested me too, but the future is not as clear cut as the cookie-cutter plan of getting your DDS, becoming an associate, and opening or buying into your own practice. Its pretty widely thought that an MBA is complete overkill for running a solo practice, so things you could do with it... go into consulting, work for an insurance company, corporate dental, become an exec in a dental supplies co, financial officer of a dental school etc. For more info go to md-mba.org.
