You need to choose "master" which is the computer connected to the keyboard/mouse you will actually use. You can later put away the mouse and keyboard of the other computer in order to avoid getting confused. The "master" must be an X-window machine but the "slave" can be anything.

After you execute these steps and "go west" off the screen on the master your keyboard and mouse events will be directed at the slave.¹

You need to install "x2x" and "x2vnc" on the master.

"slave" is also X-window.

From the slave log in to the master using ssh X forwarding

    slave:$ ssh -X luser@master

Then run "x2x" on the master via this ssh session as follows

    master:$ x2x -from :0 -to $DISPLAY -west &

Here you replace west with the appropriate direction (west equals left and north equals up) in which the monitor of the "slave" is.

"slave" is Linux in "console" mode.

On the slave you run "linuxvnc"

    slave:$ linuxvnc &

This will give you a port number (usually 5900) which you must use below. I'll use 5900 as the port since that is standard. Next start an ssh tunnel to the master.

    slave:$ ssh -f -N -R 5900:localhost:5900 master 

Finally on the master you run

    master:$ x2vnc -west localhost:0 &

"slave" is "OtherOS".

On the slave you need a VNC server like "WinVNC" for Windows and "OSXVnc" for Mac OS X. You also need "ssh" unless you allow the VNC server to accept connections over the net (bad security). You then follow the same sequence:

1. start the VNC server on slave
2. start the ssh tunnel on slave
3. start x2vnc on master.

Hope this magic helps someone.