Summer classes can be very intense, depending on the schedule. At the university I work for, we have two 5 week sessions and also an 8 week session. I think 10 credits is way too much to handle.
My daughter took a summer college algebra class a couple of years ago, and after spending two hours in class each day four days a week, she had at least 3 hours of homework each night. That's five hours - if you doubled that for two classes, you'd be spending around 10 hours a day if your workload was similar.