Multiple SQL DB for different authentication sources

Hi, i'm just wondering if what I have in mind is feasible. If it is, i'd really appreciate being pointed to the right direction or documentation :) I need to have 3 different SQL databases which will be used for authentication of different sites. How can I use a particular SQL DB for a particular Client ? E.G. : 1 db would be used for guest authentication and another db would be used for other internal authentication stuff. Thanks a lot !