Multiple SQL DB for different authentication sources

[Max .M]

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 !