Are multiple drupal sites cannot use same memcached with same port ?

Please refer to the README.txt that comes with the module:

If you want to have multiple Drupal installations share memcached instances,
you need to include a unique prefix for each Drupal installation in the $conf
array of settings.php:

$conf = array(
  ...
  'memcache_key_prefix' => 'something_unique',
);