I have installed two wordpress with same db and various prefixes, I have transformed with this tutorial: http://open-source-experiments.blogspot.com/2009/02/setting-up-common-login-across-multiple.html but I am unable to assign second user to the articles, there's not same roles I believe. but I am unable to use same roles on websites.

function _init () {
global $wpdb;
global $wp_user_roles;
 /* $this->role_key = $wpdb->prefix . 'user_roles';
$this->role_key = 'wp_user_roles';

However when I attempted this, i began obtaining the "You don't have sufficient permissions to gain access to this site.Inch error again. On reverting it back such as this, the login began working again:

function _init () {
global $wpdb;
global $wp_user_roles;
 $this->role_key = $wpdb->prefix . 'user_roles';
/* $this->role_key = 'wp_user_roles'; */

that might be solution ?