Users in Linux can belong to more than one group. In this case you want to create a brand new group, let's call it tomandruser:
sudo groupadd tomandruser Now that the group exists, add the two users to it:
sudo usermod -a -G tomandruser tomcat6 sudo usermod -a -G tomandruser ruser Now all that's left is to set the permissions on the directory:
sudo chgrp -R tomandruser /path/to/the/directory