At my school, I took bio 1 during the semester and bio 2 during the summer. It was fine. Now, after my sophmore year, I took organic chem 2 and physics 2 (at a different school) over the summer. I got an A in physics, but I know if I would have waited till the semester I would have gotten an A in ochem, but that's ok. It really depends on how difficult the professors are (I think anyway), how well you believe you can grasp the concepts, and how difficult the subject is overall. Bio during the summer really isn't difficult, but I wouldn't suggest taking any other classes while taking bio 1, if possible, since it's more difficult than bio 2.