Share User Login with other WordPress Websites

Hello, first of all sorry for opening a new topic but even there are many topics on this, none solves this issue with 3.8.1.

So I got 2 wordpress sites in 1 database.
Main table prefix: wp_
Second one table prefix: alt_

What I want to do is login to the second one with the users credential of the main one. What I did is add this to the wp-config.php file of the second WordPress site:

define('CUSTOM_USER_TABLE','wp_users');
define('CUSTOM_USER_META_TABLE', 'wp_usermeta');

Now the user gets logged in to the second one, but after logging-in, he goes to the page “wp-admin/profile.php” and getting the message “You do not have sufficient permissions to access this page.“.

I read many topics on this but I still cannot fix it. Tried changing the _init function by manually adding the table prefix but still nothing even though I read that it used to work on older versions of WordPress.

Any help would be really appreciated.Thanks!