Blog.okuryu : PHP の array_multisort で多次元配列をソートする 
$tmpOrderBy1 = array();
$tmpOrderBy2 = array();

foreach($dbResult as $key=>$row)
{
   $tmpOrderBy1[$key] = $row[orderby_column_1];
   $tmpOrderBy2[$key] = $row[orderby_column_2];
}

array_multisort($tmpOrderBy1, SORT_DESC, $tmpOrderBy2, SORT_ASC, $dbResult);