C and Java are similar enough in syntax that what you learn about basic program features like loops, conditionals and the like will directly translate from one language to the other.
The only problem I see is that C is a procedural language, and Java is an object-oriented language. While the syntax may look similar when looking at small pieces of code, the overall approach one would take to solve high-level problems is quite different in one language than the other. Some problem-solving skills in C won't be directly applicable in Java, and vice versa.
To clarify this a bit more, imagine that someone gave you a fragment of three lines of code, and asked you to determine its language. There are many such fragments of code which are literally identical in both languages, and many more which have only slight differences. However, if someone gave you an entire module of code, it would be readily apparent which language was used; the differences in overall structure are very large between C and Java.
However, as dduardo points out, most of these first-semester language courses are not going to address broad system-level problem-solving; they're going to focus on syntax and basic flow-control and computational tasks, which are quite similar in both languages.
If you're sharp and can manage to keep track of the (sometimes small) differences between the two languages, I don't think you should have a problem taking both classes at once.
- Warren