IMO a lot depends on the teacher. I had a brutal Micro teacher. My friend (now my fiance) took the same class as well as a&P 1st semester but did not work. He ended up w/ a b in a&P and a c in micro. Consequently, he had to repeat micro so he could get the b average in sciences required to go to our nursing school
. I took only micro and worked full time, I managed to get an A, but it was a VERY hard class. (This was a full semester class.)
The 2nd time around, however, my boyfriend had a much easier teacher. He says they only covered 1/3 of the stuff we did in our class before.
So my advice? Check w/ some bio majors or nursing majors that are done and get some advice on teachers. They can make or break you!
If you must get A's, I'd only take one since it's so compact in the summer. But if you can deal w/ B's and you have good instructors, you could probably pull it off, but be prepared to eat, sleep, and breathe biology.