2 wordpress user auth

    Install the second WP as normal into the same database as the original with a DIFFERENT table prefix. Activate the theme you want to use.

    Now edit these files...

    In wp-config:

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

    In wp-includes/capabilities.php:
    /**
    * Set up capability object properties.
    *
    * Will set the value for the 'cap_key' property to current database table
    * prefix, followed by 'capabilities'. Will then check to see if the
    * property matching the 'cap_key' exists and is an array. If so, it will be
    * used.
    *
    * @since 2.1.0
    *
    * @param string $cap_key Optional capability key
    * @access protected
    */
    function _init_caps( $cap_key = '' ) {
    global $wpdb;
    if ( empty($cap_key) )
    if (defined ('CUSTOM_CAPABILITIES_PREFIX')) {
    $this->cap_key = CUSTOM_CAPABILITIES_PREFIX . 'capabilities';
    } else {
    $this->cap_key = $wpdb->prefix . 'capabilities';
    }
    else
    $this->cap_key = $cap_key;
    $this->caps = &$this->{$this->cap_key};
    if ( ! is_array( $this->caps ) )
    $this->caps = array();
    $this->get_role_caps();
    }

    In the active theme's functions.php:
    function table_prefix_switch() {
    global $wpdb;
    $options = $wpdb->options; //Save the site 2 options table
    $wpdb->set_prefix('wp_'); //The prefix to site 1
    $wpdb->options = $options; //Put the options table back
    }
    add_action('init', 'table_prefix_switch');