You can't unlink the connections between your accounts on different SE sites, this is not supported by SE at all. I've posted a feature request to allow users to hide the connection between accounts, but that hasn't been implemented and I doubt it will be implemented in the near future.
You could theoretically create a completely separate account with a different Email and using a different OpenID provider, and only use that account on specific sites. This is not against the rules, but there are some things you have to be careful about.
There are some mechanisms that can trigger account merges, for example logging into a second account while being logged into your first one. You have to avoid those mechanisms, to be sure I'd use an entirely different browser for the second account.
You account also must not interact at all, but if you don't use them on the same site this should be no problem.