How to prevent multiple logins for administrator?
Session Limit allows to put restriction on session based on user role, it could be admin role also, with an exception that it does not restrict user with uid 1 which is kind of rescue point in case you misconfigure the module.

I would say instead of finding any module or building any custom solution which can put restriction on uid 1, you use different account with admin role which this/any module can handle gracefully. 

As a thumb rule I don't put any restriction on uid 1.

If you see the code of session_limit.module line 248

function session_limit_init() {
  global $user;
  if ($user-&gt;uid &gt; 1 &amp;&amp; !isset($_SESSION['session_limit'])) {

    if (_session_limit_bypass()) {
      // Bypass the session limitation on this page callback.
      return;
    }
    ..


It clearly shows uid 1 is left intentionally.