There are two ways to do this: set the directory to "world" writable or create a new group for the two users and make the directory writeable to that group.
Obviously making it world writeable is a Bad Thing, so the second option is preferable.
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
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
sudo chmod -R 770 /path/to/the/directory
Now only members of the tomandruser group can read, write, or execute anything within the directory. Note the -R argument to the chmod and chgrp commands: this tells them to recurse into every sub directory of the target directory and modify every file and directory it finds.
You may also want to change 770 to something like
774 if you want others to be able to read the files,
775 if you want others to read and execute the files, etc. Group assignment changes won't take effect until the users log out and back in.
If you also want (you probably do) that new files created inside the directory by one of the users are automaticaly writable by others in the group, then see here.