$array= Array
(
    "RT III" => 3,
    "RT II" => 2,
    "RT I" => 2,
    "RT V" => 1,
    "RT IV" => 1,
    "RT IX" => 5,
    "RT X" => 5,
    "RT XI" => 5,
);
foreach($array as $key=>$val){
        $data2[$val][] = $key;

}
krsort($data2);
foreach($array as $ar2){
foreach($ar2 as $i=>$v){
if($i>=2) break;
echo implode(', ',$v),"<br />";
}
}