Assuming the classes carried the same weight (ie 3 hour classes) your GPA would be a 1.5 most likely.
Ditto what somebody said, if you need help figuring this out, it's not a surprise you got a C and a D.
Getting a D and a D would be a 1.0 on the normal scale of A=4, B=3, C=2, D=1, F=0.
Now, that's pretty awful. I'm not saying I never had a semester where I got a C and a D, but at least they were in things like Calc 3 and Electromagnitism. (actually I probably got a D and an F, respectively). But now I have a more successful career than most of the smartasses in this thread. ;-) So don't give up, but seriously man - work harder next time!
Edit: If this puts you on academic probation or warning, DON'T do anything stupid like sign up for the same two classes again. (unless you have to) The key to surviving bad grades in college is to pad your schedule. If you took two hard classes and did badly, and don't think you'll do better next time - PAD YOUR NEXT SEMESTER WITH EASY CLASSES. It's better to EXTEND your stay in college than it is to get THROWN OUT. With grades like this you need to get a copy of your school's rules and regulations and study the sections on academic requirements.