success: function(data){
            var toAppend = '';
            if(typeof data === "object"){
                for(var i=0;i<data.length;i++){
                    toAppend += '<tr><td colspan="2">'+data[i]['main-asin'][0]+'</td></tr>';
                    toAppend += '<tr><td>'+data[i]['sub-asin'][0]+'</td><td></td></tr>';
                }
                $('.data-results').append(toAppend);
            }
        }

foreach($xml->Items->Item as $item){
    $items_from_amazon[] = array('main-asin'=>$item->ASIN);
    foreach($xml->Items->Item->Variations->Item as $item){
        $items_from_amazon[] = array('sub-asin'=>$item->ASIN);
    }
}

echo json_encode($items_from_amazon); //return amazon products

toAppend += '<tr><td colspan="2">'+data[0]['main-asin'][0]+'</td></tr>';
for(var i = 1; i<data.length; i++){
  toAppend += '<tr><td>'+data[i]['sub-asin'][0]+'</td><td></td></tr>';
}

PHP:

foreach($xml->Items->Item as $item){
  $new_item_from_amazon = array(
    'main-asin' => $item->ASIN,
    'sub-asin'  => array(),
  );
  foreach($xml->Items->Item->Variations->Item as $item) {
    $new_item_from_amazon['sub_asin'][] = $item->ASIN;
  }
  $items_from_amazon[] = $new_item_from_amazon;
}

JS:

toAppend += '<tr><td colspan="2">'+data['main-asin']+'</td></tr>';
for (var i = 0, l = data['sub-asin'].length; i < l; i++){
  toAppend += '<tr><td>'+data['sub-asin'][i]+'</td><td></td></tr>';
}