You can definitely enroll a student in multiple classes. However, Typesy recommends avoiding this unless you have a specific reason for the student to be in multiple classes at the same time.
1. To get started, go to the class where you want the students to be enrolled. From the Class Menu, click on Enroll Students.

2. Uncheck the box labeled Remove from other classes when enrolling in new.

3. Once done, select the students to enroll and click Enroll Selected Users.