> Store the uid of each provider in twitter_user and facebook_user. The first used will define name and nickname.

OK, that's fine, I was thinking of having a unique field, and that's where I thought of duplication.

 > I don't know if you have plans to manage connections from user settings.

No, I'd keep it simple. Letting the user be consisten with what he signs in. If the same user logs in with both accounts, for FWC will be two different accounts.