The screen command is usefull for mirroring a screen between two terminal sessions.
1. In the home directory of user1 create the .screenrc file that has the following lines.
$ vi .screenrc
2. As root change the permissions of /usr/bin/screen to 4755. This will add the SUID bit.
$ chmod 4755 /usr/bin/screen
$ l /usr/bin/screen
-rwsr-xr-x 1 root root 308896 2007-05-03 07:44 /usr/bin/screen
3. As user1 run “screen”
4. From a different terminal as user2 run “screen -x user1/”. The / is necessary
$ screen -x user1 /
Now both users can see the same screen and either can type and it is visible to each user.