add media to custom folder

Hopefully its a simple question for more experienced Wordpress programmers.

I am using

<?php media_sideload_image($file, $post_id, $desc); ?> 

However this function adds the media with a folder structure that is created by WP. I want to use a custom folder. I read on How to save media files under custom folder without changing wp-config.php or changing media settings about applying a filter. But then I wonder what happens when two users at the same moment are trying to upload a file.