Honestly, I'm doing fine right now. I'm taking my BIO201 midterm next week and I'm currently at a low A. I spend about two days every week on my anatomy. I'm in my fourth week of CHM130 with lab and have high A's in both the lecture and the lab portion. It's really not all that hard, I spend an average 8 hours reading and studying the material and then another 3 hours on the lab, that totals to one day. Then I'm almost done with ENG101 (English is my third language) and I have another high A in this class. I spend about 4 hours every week on my English assignments as well.
When you add this all together, this is my week.
Monday: Work 12h shift.
Tuesday: Study for BIO201.
Wednesday: Study and do lab for BIO201.
Thursday: Study/Lab for CHM130.
Friday: Work 12h shift.
Saturday: Turn in BIO201 assignemnts and do online quizzes. I usually take the rest of the day off.
Sunday: Spend the morning on ENG101. Take the rest of the day off again.
It's actually not as crazy as you would think. I do expect to spend some more time on the CHM130 in the future classes, I anticipate it will get harder. So I can put those extra hours in on Sunday. Besides work, I only have a husband and no children. This helps as well.
I really think it's possible to combine two science classes, especially if you can work part-time and are able to spend time on your classes. Remember Rio Salado does advise to spend about 16hrs/week for 4 credit hour classes and 12hrs/week for 3 credit classes.
Good luck! I hope everything will work out!! Also, keep an eye on the MCC NS newsletter. It comes out on Jan. 1, 2008. They might turn some pre-req's into co-req's then, which could shorten your pre-req schedule!