How is the one file 'hidden' in the other? You could, for example, maybe like

Read up until the hidden file. Remember where that point in the file is. Read the hidden file into a buffer. Then read the rest of the file into another buffer. Then seek back to the point where the hidden file was, write the rest of the file buffer to that place, and set the EOF after it.

Edit: If you just don't know the API's and didn't need conceptual stuff I can list those