Alternate your subjects for 2 reasons:
1. You're not likely to remember much of the first subject you study you study your subjects in bulk. I.e. 3 months it's not likely you'll remember a whole lot from bio if you went from chapter 1-10 in 10 days.
2. The MCAT is about general concepts and relationships. You want to layer your study because the test is going to layer it's sections. This is one reason, if not the only, why each science section on the test is a combination of topics (35% x, 15% y, etc.) instead of one physics sections, one biology section, etc. The test is integrated and so you should study accordingly.
