yes it does get really frustrating and it amazes me how much patience Jason and the team have when dealing with php numpties like me.;-)
The database is imaterial to your issue, the users registration only creates one table called users. You can out that table in any database you have and access it by adding connections details in the localhost.php
Ok you may be getting confused with the term local host as that ususally applies to your own machine, it is in fact just a file name it could be connections, xyz or 123456. What is important is that everything that WA do uses the file name local.php to store the connection details. I always create three connection files localhost_local, localhost_remote and localhost.that way I can just overwrite the localhost one when uploading. I also make the connection files read only ( stops the oopppps wish I hadn't done that error).
At the top of everypage the connections localhost.php is called so that should not be an issue once you get the settings right.
If you have two tables called users that may/will cause havoc unless the table structure is identical.
I suggest you give that some thought and come back with more information about the table structure.