(This is for posts, not pages - the principle is same. The permalink hook is different by exact use case)
I just had the same issue and created a more convenient way to do that - where you don't have to re-edit your functions.php all the time, or fiddle around with your server settings on each addition (I do not like both).
TLTR
You can add a filter on the actual WP permalink function you need (for me it was post_link, because I needed that page alias in an archive/category list), and dynamically read the referenced ID from the alias post itself.
This is ok, because the post is an alias, so you won't need the content anyways.
First step is to open the alias post and put the ID of the referenced post as content (and nothing else):
Next, open your functions.php and add:
function prefix_filter_post_permalink($url, $post) {
// if the content of the post to get the permalink for is just a number...
if (is_numeric($post->post_content)) {
// instead, return the permalink for the post that has this ID
return get_the_permalink((int)$post->post_content);
}
return $url;
}
add_filter('post_link', 'prefix_filter_post_permalink', 10, 2 );
That's it
Now, each time you need to create an alias post, just put the ID of the referenced post as the content, and you're done.
This will just change the permalink. Title, excerpt and so on will be shown as-is, which is usually desired. More tweaking to your needs is on you, also, the "is it a number" part in the PHP code is far from ideal, but like this for making the point readable.