Auth keeps a login hash internally, and rejects a re-login via Auth::check() if the hash doesn't match. This check is disabled when 'multiple_logins' is set to true.
This is checked in the auth\login\simpleauth.php class, in the perform_check() method. Perhaps you should debug that to see if it does what it must do.
What is exactly the problem, because I don't see any reason what that should not work.
The simpleauth driver does
// return true when login was verified, and either the hash matches or multiple logins are allowed if ($this->user and (\Config::get('simpleauth.multiple_logins', false) or $this->user['login_hash'] === $login_hash)) { return true; }
which means the login is valid if a user was found, and either multiple_logins is true or the login hash matches.
If you don't use the "multiple_logins" setting, the login-hash is used to prevent multiple logins. If you do use the setting (i.e. set it to true), the login-hash isn't used, and multiple logins of the same user is accepted.