[]This would in principle be enough, but since we are getting quite a few people who are getting confused with two seperate logins combining it into one registration process would be preferable.
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As i said before, it won't be done with one registration process. Making a registration page that writes the data in the ubb.threads database and in your members area database isn't difficult. The difficult part is the interacting of the two databases. You need to update your members database also, when someone changes his email address within threads and vice versa.
So you'll have to enhance several threads scripts aswell as your scripts of your member area. I don't recommend doing that. Maintaing the same data in two databases always produces inconstiences and errors.
Using the same usertable for threads and your members area would be the better way, in my opinion.
Depending the scripts that you use in your member area, it would be less work and a cleaner solution.