Think you're going to need two certifications, and you'll then have to put up with an exception which will need to be overridden for everyone that gets added:

Certification X:

• Active for: 5 years
• Contains Course X only

Certification X or Y:

• Active for: 1 year
• Contains both Course X and Course Y

When someone does Course X, they get a "tick" for both certifications. When they complete Course Y, they only get the "tick" for "Certification X or Y".

It's not a beautiful solution, but it allows you to track both things you need to and keeps things fairly simple from the user perspective. All you'll need to do is put some guidance in the Certification description to explain, and maybe put Course X into a different section to Course Y with an appropriate title telling staff to check their compliance with Certification X.

Dan