For med school, undergrad professor letters are expected
For B-school, letters from supervisors/employers tend to be more relevent and timely
Since I assume you are applying straight from undergrad into an MD/MBA program, you may be forced to use only undergrad professor letters to cover both. I would recommend getting any employer or business-related letters you can from prior work experience/summer internships, etc, since they tend to carry more weight in B-school than others. However, if you did something like take a purely business-related course in undergrad and it was recent, a letter from that professor may also be appropriate. For my B-school application, my letters came from summer internship employers, undergrad student group advisors, and MBA professors (I had taken an MBA class as a first year med student).