settings.php

<?php
//...

$db_prefix = array(
  'default'        => '',
  'authmap'        => 'databasedosusuarios.',
  'sessions'       => 'databasedosusuarios.',
  'users'          => 'databasedosusuarios.',
);

//...

$base_url = 'http://subdominio.dominiodositepai.com';

//...

$cookie_domain = 'dominiodositepai.com';

// ...

?>