At the bottom of the list of existing enrolment methods there is a drop down menu "Add method". Select "Course meta link" from the menu.
Find the course you would like to pull the enrolments from in the drop down menu, or search the course name, then and click "Add method"
Done! Now all students from the other course will also be enrolled in yours. When they are unenrolled from the other course, they will also be unenrolled from yours.