<?php
$array1 = array(array("a"=>"aaa","b"=>"bbb"),array("x"=>"xxx","y"=>"yyy"));
$array2= array(array("c"=>"ccc"),array("z"=>"zzz"));
$i=0;
$j=0;
foreach($array2 as $single){
    foreach($array1 as $multiple){
        if($i == $j){
            $keys = array_keys($single);

            print $key = $keys[0];
            $multiple = array_push_assoc($multiple,$key,$single[$key]);
            $array1[$j] = $multiple;
        }
        ++$j;
    }
    ++$i;
}

var_dump($array1);

function array_push_assoc($array, $key, $value){
$array[$key] = $value;
return $array;
}
?>

carray(2) {
  [0]=>
  array(3) {
    ["a"]=>
    string(3) "aaa"
    ["b"]=>
    string(3) "bbb"
    ["c"]=>
    string(3) "ccc"
  }
  [1]=>
  array(2) {
    ["x"]=>
    string(3) "xxx"
    ["y"]=>
    string(3) "yyy"
  }
}