<?php

$array1 = [
  '1' => 285.52,
  '2' => 427.76
];

$array2 = [
  '1' => 123.44,
  '2' => 48.32
];

foreach($array1 as $key => $val) {
    $mergedArrays[$key] = array('owed' => $val, 'payed' => $array2[$key]);
}

print_r($mergedArrays);