Users should not have two roles in the same course since one role's permissions will override the other. If a student is officially enrolled in the course, providing the student with the TA role will allow the student to his/her classmates' roster photos, grades, and personal information which violates privacy policies. Officially enrolled students should not be added with the TA role.
In addition, if an officially enrolled student is also added as a TA, the student will no longer be able to perform student functions such as submitting assignments and signing up for appointment slots. Instead, they can see assignment submissions of his/her classmates, etc.