You can use the screen tool to share a console between two users - this is useful for remote collaborative development, and doesn't require nearly as much bandwidth as a desktop-sharing protocol like VNC.

For this document, I'll use the example of user 'Alice' creating a shared console for 'Bob' to join in on.

setting up the system

You'll need a local installation of screen, and make a few permissions changes to allow multi-user mode. The screen binary needs to be setuid-root, and the /var/run/screen directory needs to be at least mode 0755.:

alice $ sudo chmod u+s `which screen`
alice $ sudo chmod 755 /var/run/screen

Note that creating setuid-root binaries on a machine is usually not a good idea. I'd advise against doing this on a machine that hosts untrusted users.

setting up the hosting session

User alice needs to run screen and turn 'multiuser mode' on, then allow user bob to access it. 

alice $ screen

And then the keystrokes (C-a is ctrl + a):

C-a : multiuser on
C-a : addacl bob

Now the session is available to Bob to connect - all he needs is the session id. This is shown by 

alice $ screen -ls
and will be something like alice/1942.pts-3.localhost.

connecting to the existing session

Once Alice has started screen and given Bob the session-id, he can connect with screen -x:

bob $ screen -S session-id -x

Then Alice's console will be available to Bob. Both users will see the output on the console, both are able to input characters.