<?php
    $array= Array
(
    "RT III" => 3,
    "RT II" => 2,
    "RT I" => 2,
    "RT V" => 1,
    "RT IV" => 4,
    "RT IX" => 4,
    "RT X" => 4,
    "RT XI" => 5,
);



foreach($array as $key=>$val){
        $data2[$val][] = $key;

}
krsort($data2);

    $i = 1;
    $finish = false;
    foreach($data2 as $array2){
            foreach($array2 as $value){
                    echo $value.' ';
                    if($i >= 2){
                            $finish = true;
                            break;
                    }
                    $i++;
            }
            if($finish) break;
    }
harusnya outputnya :
RT XI
RT IV ,RT IX, RT X