Not without searching, which you all can do too. I only say what I said above because I researched into how to join two user database tables because I was thinking of linking two Joomla sites together. From what I read at that time, and from what I know now as a PHP/MySQL Programmer, you would have to have two different connections to two databases. Then, you have to check the entered username/password from both tables, choose which table to use the user information from, etc. I guess if you wanted to keep it more on the simple side, you might be able to do it without too many problems. I'm sure there are others who have actual practical experience in this, so I'll let them give concrete details
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See Login in a diferent site


I think you didn´t understand my real situation: I want to logon a joomla! site of a no joomla site!!!...is diferent...in your case maybe with the module J!login can resolve the problem!!!