While these tags do indeed have a lot in common, and there is indeed overlap, I think they focus on different aspects.
Often someone would want to ask specifically about attributes of certificates, or applicative use and storage, for e.g. asymmetric encryption (sans PKI).
Or some questions would be specifically about how to secure or restrict a certificate-authority, without going into the wider PKI-implementation details.
Or they're looking at the user management end of public-key-infrastructure.
So yeah, they're definitely close and similar, and often you'd be using more than one (perhaps even as an incomplete subset of another), but I don't think they are identical, or close enough to merge.
Btw, regarding "certificate authority" vs. "certification authority" - both Google and Bing return twice as many hits for the first than the second. Wikipedia has both registered on the same article, but has "certificate authority" as the canonical link. so, not quite conclusive, in either direction - but I would not be against a synonym from the 2nd to the 1st.