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.
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.