So I guess that's a "yes" - they both need ftp access to the same directory. You are correct, that each user has their own directory, and this is the way it is supposed to work on DH.
There is more information about this on the DH Wiki:
How do I share ftp access with another FTP user?
The easiest way to accomplish what you want to do is to have them share the same FTP/SFTP credentials (act as the same user). If this doesn't meet your needs, you will need to either implement some scripted solution (a filemanager program, CMS, etc.) or cobble something together using unix groups, specialized file and directory permissions, and possibly cron jobs for moving files around (not a pretty picture!) .