Unless you have the desire and ability to spend every waking moment (yes, even in the shower) studying for your two classes, don't even consider taking them at the same time. But that's just my

. Normally, I'm a 4.0 student
BUT this summer I am taking Micro and Stats along with World Geography. There is so much new information and memorizing that has to be done...aaaggghhhhh! I just can't remember it all at the same time. I made a bad decision by taking too many memory intensive classes and it will affect my GPA which is really important for grad school.
If you are making a plan for the future and not a plan for this summer, I suggest rearranging things a little bit to avoid this situation. It's great that you are making a plan. It really does help.
Good luck.