There is a workaround: make the multiple parents children of one parent if they’re not already, then use that parent to list just the grandchildren, almost like a ‘post_grandparent’.
Found this code elsewhere on the support forums, seems to work for me:
<?php
echo 'get pages';
$parent = 93;
$args=array(
'child_of' => $parent
);
$pages = get_pages($args);
if ($pages) {
foreach($pages as $post) {
setup_postdata($post);
if ($post->post_parent != $parent ) { ?>
<p><a href="<?php the_permalink() ?>" rel="bookmark" title="Permanent Link to <?php the_title_attribute(); ?>"><?php the_title(); ?></a></p>
<?php
}
} // foreach($pages
} // if ($pages
?>