You may want to read up on
Java packages:
http://java.sun.com/docs/books/tutorial/java/package/index.html
In short, the package structure needs to match the directory structure, so both classes need to be in the same directory since they're in the same package, and that directory must be named "yemi".
Put both .java files into C:/SCJP/yemi, and then change into the directory C:/SCJP. Then the compilation should work (javac yemi/Moo.java and javac yemi/Zoo.java). You can also compile both classes at the same time through "javac yemi/*.java".