Just setup a third user ("adduser [--disabled-login] user3", then share his home directory: make both users members of the third user's main group "adduser user

Just setup a third user ("adduser [--disabled-login] user3", then share his home directory: make both users members of the third user's main group "adduser user1 user3 && adduser user2 user3" (assuming you don't want to alter both user's main groups for privacy), then "chmod g+wx /home/user3" and have a cron job enforcing all files in the shared directory belong to the user3 user/group and are writable by the group "* * * * * root chown -r user3:user3 /home/user3 [possibly add: && false || chmod -r g+w /home/user3]". Then create (soft|hard) links to the third user's home dir if you like. This way you retain each user's privacy, since main groups aren't changed, don't need to run custom scripts each time, and don't have multiple copies of the shared files. The only (IMHO minor) issue would be that newly shared files might only be accessible for the other user 30s later on average, and 1 minute at most. The cron job might be converted into a daemon monitoring new files appearing in /home/user3, but that is left as an exercise for the reader :)