Read up on UNIX permissions:
http://www.zzee.com/solutions/unix-permissions.shtml
The best way to do it, put both users in the same group, then give the files/directories group write access. You'll also need to make sure the files/directories are owned by said group.
Running FTP over the internet with cleartext passwords is sorta okay, as long as you're not using system accounts for your FTP logins. Last I checked, ProFTPd allows you to use the system accounts, or store the info in files or a SQL database.
This would be okay, provided the passwords used for these users are not the same as any other passwords.
However, passwords in cleartext is a *bad* idea.