x2x allows the keyboard and mouse on one ("from") X display to be used
to control another ("to") X display.  Since x2x uses the XTEST
extension, the "to" X display must support XTEST.

In the default interface, x2x puts a window on the "from" display.
This window is labeled with the name of the "to" display.  Keystrokes
typed into this window go to the window on the "to" display that has
the input focus.  Clicking on the x2x window causes the mouse on the
"from" display to control the cursor on the "to" display.  Perform-
ing a subsequent multiple button click on the "to" display returns
control to the "from" display.

If the -east or -west options are specified on the command line, x2x
starts up with a different interface.  When the mouse moves to the
(east or west) side of the default screen on the "from" display, the
cursor slides over to the "to" display.  When the mouse returns to to
side of the "to" display that it entered, it slides back onto the
"from" display.

Unless the -nosel option is specified, x2x relays X selections from
one display to the other.