Hmm, interesting. I may have a unique perspective on your interests because I went to law school, worked for the bureau and am now in medical school -- it certainly made my interviews interesting!!!
First of all, if you want to be an FBI agent, being a lawyer will certainly help. The bureau likes to hire lawyers and accountants. I don't think being a MD/JD would help you very much in the application process because it is so rigid. You may get you application moved higher in the stack of those that get tested or interviewed. Actually, doing it may hurt you. The bureau has strict age guideline, they don't hire those over 37 unless they have prior federal service -- so keep that in mind.
If you want to work in the FBI labs, I'm sure that being a pathologist would help, but I don't remember any of the lab guys being MDs. Most were PhDs or Masters, but I'll bet there were some MDs. Remember, though, that the salaries will suck and you won't be doing investigations like an agent would. To the best of my knowlege, all of the profilers were Agents, but there may have been a psychologist or two around.
If you truely want to be an agent, med school is not your best route. In any event, the application process is long and to a certain extent arbitrary. Only about 1 in 100 who apply get a job.
Ed