<?php

$array_final = array();
$array_atual = array (

    'id_item' => array('1', '3'), // 'id_item', 'id_item'
    'cantidad' => array('15', '10') // 'cantidad', 'cantidad'
);

foreach($array_atual['id_item'] as $key => $value){
	$newItem = array($value, $array_atual['cantidad'][$key]);
	array_push($array_final, $newItem);
}

print_r($array_final);