Forumlar / Cotonti / General / Trouble with using same user table on two sites

It reads what you tell it to read :)

In your config file you can set the tables to be named anything. The first table could be xxx_auth and the second could be modz_bbcode. Doesnt really matter, as long as they exist.

What you're missing here is that this is not a db-wise prefix, it's on a table-basis. And the prefix is nothing more than a unique-fier.

Tip: to not lose your db make sure you back it up first and on every step.