<?php

$teste1 = array(
      1 => array( 1 => 'teste1', 2 => 'teste2', 3 => 'teste3'),
      2 => array(3 => 'teste4', 5 => 'teste5')
);

$teste2 = array(
      1 => array( 3 => 'teste6', 4 => 'teste7')
);

function uniqKeys($arr1, $arr2)
{
    foreach($arr2 as $key => $value)
    {
        $arr1[$key] = array_key_exists($key, $arr1) ? array_merge($arr1[$key], $value) : $value;
    }
    
    return $arr1;
}

print_r(uniqKeys($teste1, $teste2));