Sorry but this could be a pain. I believe students would have to be enrolled or allowed access to both courses for them to access the url.
Ideas: put your files in a "common" course, think of it as a library, then the url will not change in the Moodlesite. Not sure how many used this concept. Some put these files on the site level folder, others have a course for a department and enroll all students in a department. The idea is to put documents or lessons which are common or useful to one or more courses/teachers.
Learn a little more about
Meta course, where enrolment in one automatically enrolls the student in another other. See the first example on that page, students in any one of many different courses are automatically enrolled in the meta course.
Hope this helps. Chris