I have a hobby site and a professional site, and right now my OpenID is only linked to one. Is it possible to link to both? Is this a good idea?
Your OpenID is provided through an OpenIDProvider. So, it doesn't matter how many different sites use it. If you decide that both sites use the same provider, that is fine. In fact, that's part of the design of OpenID.
Think about Google. Google is an openIDProvider and is used by MANY MANY websites to authenticate their users for them. That's how it's designed.