I took the MCAT before I took Biochem II/Physics II and I did fine (129/129/127/131), but looking back, it was not the best choice I could have made. I had to teach myself a ton of material, and even then, it was more just memorizing the equations that kept popping up and praying the test wouldn't be super hard.
I'll put it like this: my month of January before the test would have been much less stressful if I had put it off until I had at least started the classes.
i took the MCAT concurrently with biochem and physics II and it was fine. The biochem on the MCAT is totally teachable with the proper set of books and dedication