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\', \'/\');