I'm in a masters, currently, and it is in a science (Pharmacology and Toxicology) and thesis based. Basically, when I'm not in class, I'm in the lab doing research for 20+ hours a week.
If you want to do prereqs while in your masters, you should have a good relationship with your graduate adviser (and the department) because you can't take any class you want 100% of the time. Plus, if its in a science, you'll probably have to do a thesis, and that will take up a big chunk of time. See if a coursework masters is possible if you're interested in pursuing this route, otherwise you may not finish your thesis on time and you'll be there for 3 years.