sharing authentication within two drupal distributions
This method comes with a list of caveats, but it's possible to simply share the users table across multiple installations via a table prefix array in settings.php.


  Example: Sharing Users
  This example shows how to share users. The
  users from a master site prefixed with 'master_' are shared to a slave
  site prefixed with 'slave1_'.


$db_prefix = array(
    "default" =&gt; "slave1_", // the prefix for tables that are not shared.
    "users" =&gt; "master_",
    "sessions" =&gt; "master_",
    "authmap" =&gt; "master_",
    "sequences" =&gt; "master_",
    "profile_fields" =&gt; "master_",
    "profile_values" =&gt; "master_",
);