Hi Liz
It is not recommended that the same course is used in more than one certification as when the course is reset for a users who is recertifying this will also reset the course in any other certifications that they are assigned to that has this course .
You will receive an exception if you try to do this and the users won't be assigned to the certification unless you choose to assign them. You can choose to assign if the user is only ever going to be assigned to one certification. if this isn't the case then create a copy of the course for each certification.
Regards