Until a doc answers...
There are people who get double-specialties. However, they generally do so one at a time. It's not too easy, though. Remember that residency spots are limited, and med students must compete for spots to get a slot in the residency they want. If you decided to pursue a second specialty, you'd be competing against medical student graduates, which is going to be an uphill battle.
Remember too that anesthesiology is generally a pretty competitive spot. There's good money for, what at least pre-meds believe, is an "easier" job than other medical specialties offer.
I wouldn't worry too much about that now. You have a LOT of things to get done before worrying about this. Most people think they know what they want to do until they actually go through med school, and suddenly find themselves interested in something completely different. Especially as a high school student, it's almost impossible to know what your interests are without having any experience with the field, or even the science behind the field.
I'm not trying to invalidate your question, but rather say that in a couple of years, you'll probably be so overwhelmed with the process of trying to get into medical school that your attempt to complete two specialties will seem REALLY far away.