It sounds like you have a course completion certificate, UoF, which has an expiry date 2 years after date of issue, & that participants need a valid certificate to take further, dependent courses. I hope I've understood that correctly.
One possible solution is to allow participants to enrol on the course but then require them to submit a valid UoF certificate in order to proceed to the rest of the course material. You could make it automatic (on submission of the certificate) & tutors manually follow up afterwards, which is faster/immediate, or if you prefer, require tutors to approve the certificate before they can access the rest of the course.
I hope this helps!

