I have put together a web site using two open source projects ( Open Realty & GeoClassifieds Light ), and have styled them both to be identical with one another. Now I want to integrate the user registration and Login. I want to make one form that will register a user to both sections of the site. Also, I want one Login that will be recognized between both sections. This way, the user can register once and login once, but can manage their account on both sides.

Any feedback on the direction I should move in to make this possible, will be greatly appreciated.

I don't know the internals of their structure, but this is going to require some delicate coding on your part. As a summary:

• Take a snapshot of each database.
• Add a user to each database.
• Take another snapshot and compare the before-and-after versions of each database using something like WinMerge.
• Create your own account registration which properly inserts all records that need to be inserted for each app (that's what the difference comparison you performed above will let you do) or modify each of the account registrations to insert fields into the "other" database.
• Now, login to each site and echo out the SESSION variables. If you're in luck, these are done using file-based sessions. If not, it gets even more harry.
• Go to the login processor for each site and modify it to set the session variables for the other application.
• Change the profile management features of each app to update the other table as well.

There are other ways you can do this, but the method described above is probably the easiest.