if node property = x
  use this sort criteria
else
  use this one

/**
 * Implements hook_views_query_alter().
 */
function HOOK_views_query_alter(&$view, &$query) {
  if ($view->name == 'view_name' && $view->current_display == 'page') {
    $query->orderby = array(
      array(
        'field' => 'CASE WHEN node.property = x THEN node.y ELSE node.z END',
        'direction' => 'ASC',
      )
    );
  }
}