Yes this is possible. You could set up the user validation within one website and have it act as the clearing house for user data. There really is no need to have both databases talk to eash other unless you plan to keep separate user data.

A simple connection to the database holding the credentials will take care of the matter. Assuming the databases are on the same server it is a piece of cake. If they reside on different servers it will work almost as easily. Just make sure the chosen database can handle remote connections.

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I use Country IP Blocks as added security for my networks and servers.