The easiest way is to duplicate the exam to get two identical exams. Then you can use the new exam with the other class.

1. Locate the exam in the exam list.
2. Click on the three dots to the right and select "Duplicate the exam"
3. Name the exam.
4. Click "Create an exam"

Now, you can find the exam in the exam list. Here, you can read more about preparing and conducting an exam.