Log in to all contexts simultaneously

 // use the credentials you got
 $_POST['username'] = $username;
 $_POST['password'] = $password;

 // in another context
 $_POST['login_context'] = 'contextNameHere';
 
 /* send to login processor and handle response */
 $response = $modx->executeProcessor(array(
   'action' => 'login',
   'location' => 'security'
 ));
 if (!empty($response) && is_array($response)) {
   if (!empty($response['success']) && isset($response['object'])) {
// logged in successfully
  }
 }