multiple wordpress installation with shared usertable on an different database

After writing those two lines in wp-config.php, your users data will be shared between both installations, but both installations will also need to have the same cookie parameters, so in wp-config.php file of your second installation, modify the COOKIE_DOMAIN like this: define(‘COOKIE_DOMAIN’, ‘.{yoursite.com}’); //replace with URL of first site define(‘COOKIEPATH’, “https://wordpress.stackexchange.com/”);