Excluding / including search results (function.php)

functions.php

It works:

function search_exclude($query)
{
if ($query->is_search)
{
$query->set('p','52');
}
return $query;
}
add_filter('pre_get_posts','search_exclude');

It doesn’t work:

function search_exclude($query)
{
if ($query->is_search)
{
$query->set('p','52,53');
}
return $query;
}
add_filter('pre_get_posts','search_exclude');

If I’m doing something wrong?