<?php

function mesclar(array $arrX, array $arrY)
{
    $z = array();
    $chaves = array_merge( array_keys($arrX), array_keys($arrY) ); //Pega as chaves de ambas arrays
    
    foreach ($chaves as $chave)
    {
        if (isset($z[$chave]) === false) {
            $z[$chave] = array();
        }

        if (array_key_exists($chave, $arrX))
        {
            $z[$chave]['X'] = $arrX[ $chave ];
        }

        if (array_key_exists($chave, $arrY))
        {
            $z[$chave]['Y'] = $arrY[ $chave ];
        }
    }

    return $z;
}

$arX = array('AAA' => 173.696, 'BBB' => 72.436, 'CCC' => 142.692);
$arY = array('AAA' => 127, 'DDD' => 72.333);

$novaArray = mesclar($arX, $arY);

print_r( $novaArray );