I know it's a bit early, but I'm already thinking ahead to which specialty I'd like to pursue once I get my MD. I really want to pursue either anesthesiology or emergency medicine, and I'm torn between the two. When the time comes for me to choose my specialty, if I still can't decide what I want to do, can I do 2 separate residencies and specialize in emergency med and anesthesiology?
As penguin said, theoretically you could, realistically and practically, no you wouldn't.
First, it's way too early to focus in on a specialty. Your mind will change when you actually get to med school rotations and see what these folks actually do all day. Some fields sound more glamorous until you get there, others are sleepers that you never thought you'd like until you try it. So don't worry about picking a field now.
Second, in order to specialize, you need to do 3+ years of residency after med school for any specialty (3 for EM, 4 for anesthesia), and maybe a fellowship for 1-2 years after that, depending on the field. So we are talking about an extra 4-5 years after you complete your first residency before you will be board certified in another and actually ready to earn a decent living.
Third, there are pretty much no jobs out there that require one person having both of these specialties. You won't be making yourself more marketable. Meaning you still have to choose which one you want to practice in, you are just delaying the decision a few years.
Fourth, both specialties have continuing education and careerlong periodic recertification testing, so you will be dooming yourself to stay on top of two fields, instead of one for the rest of your career, which isn't a small task.
Fifth, as mentioned in the prior post, the government funding for residency runs dry on you by the time you want your second residency, so programs that will take you are fewer, because you don't come with funding.
So basically, no you won't be doing this. You probably won't end up liking either of those fields in the many years it's going to be before you have to make a decision anyhow, though.