Lots of things to add here.
1. I wouldn't bother with an MD-JD program. The only people who have both degrees ought to be people who make career change or people who take some bizarre career twist that might utilize both. There used to be whole websites devoted to why this is just a moneymaking sham designed by schools to maximize tuition by providing minimal value. You wont earn more having both degrees, but certainly will incur a lot more debt. The short answer -- the number of jobs where you would ever need both degrees are almost nonexistent. You won't have an easier time doing medical malpractice law as a lawyer with an MD. You not be a better or more sought after expert witness as a doctor if you have a JD. There are maybe a tiny handful of jobs I could conceive of where you could actually usefully use both -- as a professor or in health policy. But in most cases you'd have to work very hard to sell to an employer why having both was advantageous. And employers would still eye you suspiciously that you are planning to jump ship to the other career. No law firm will touch a guy they think is really a doctor waiting for the right doctoring job to emerge. So basically you are going to have to choose -- to only practice in one world, and can save yourself a lot of tuition and time if you pick one. As I'm a living testament you can change your mind down the road, but I still wouldn't try to straddle the two fields.
2. Criminal law is one if the legal subspecialties that would be hardest to pair with medicine. If you were to combine the two fields (which as mentioned is not a good plan), health law, health policy and contracts/business (because medicine is a business) would be the areas with the most cross over. "criminology" is less of a lawyer term and more of a psychology/ law enforcement term, so if the study of criminals is what you are getting at, law school won't be what you had in mind; in law, they are just clients and we don't get into the psychology of what they did, and frequently don't even want to know if they even did it, we just help them through the justice system, much as you'd help a Business client navigate federal regulations.
3. As mentioned above, don't do a combined degree just to put off deciding what you are interested in. Take a year off to do peace corps or something like that if you cnt decide and need to find your calling. Dual degree isnt a good stalling technique because you rack up debt. Med schools actually like to see people who did peace corps or explored other interests before med school. Or at least did extensive shadowing to figure out what they liked. Get a job at a law firm, volunteer at the local ED.
4. I'm not sure you ever expressed why you think you are interested in both medicine and law. While both jobs have overlap, there ought to be articulable reasons why you feel one or the other is a Better fit. don't try to figure this out during professional school -- figure it out first.
5. Have a plan. If you can sit down and say here's what I want to do and here's why I need both degrees to get there, nobody can really question your decision. Don't take the field of dreams "if I build it, they will come" approach to your career. Doesn't work that way. The MD-JD programs capitalize on people without a clue, but the value added just isn't there. As mentioned, there are other combined degrees, like MD-PhD that actually might provide value. This one doesn't.
6. In term if job market, law is tighter, but there are actually medical specialties where longer job searches are becoming more common. To be honest, I don't know how much better it is to do 4 years of med school and 4 years of residency/ fellowship to spend six months looking for the right position, as opposed to spending 3 years of med school to spend a year looking for the right position. The market in law is awful, but still somewhat exaggerated on SDN. It helps to go to a top ten law school to get a big law job, but that's hardly the only path that works -- the top grads at all the regional law schools also get into the big to midsized firms. And if you are any good at marketing, you can even make money as a solo or small firm -- I know a lot if people who carved out nice niches. And in criminal law, which OP clims he likes, the big firm isn't really usually the target employer -- you'll be looking at PD, county prosecutor, DA and small firm practices to do that anyhow. The competition really isn't that bad for this end of the legal market, and the guy smart enough for med school would not have difficulty landing a job in this area. You would, however have a Tough time servicing a dual degree student loan debt with this kind of job.
Seriously? Did you not just read all the previous posts...
