Would a background in psychology or criminology be more useful for a Medical Examiner? At my college of choice, in the forensic program it is required to get a double major.

IF your long-term goal is to be a medical examiner (pathologist is another name for this role), you will need to obtain an M.D.

The educational preparation for a medical degree is an arduous and lengthy process. You should attend an undergraduate program that has a strong pre-med program with an established track record of getting its graduates into medical school! Those successful programs usually have academic advisors who specialize in preparing undergraduates for medical school admission.

While a psychology major will be very useful in preparing for the MCAT (Medical College Admissions Test), a second major in criminology will not be helpful.

Why, you might ask?

You also need significant coursework in the life and physical sciences (biology, microbiology; inorganic and organic chemistry; physics).

A forensic, criminology major is not likely to include all of the coursework I have identified in the paragraph above.