x2x joins a pair of X displays together, as if they were a single
multiheaded display. The pointer can be warped between displays,
or, depending on how you start x2x, can slide from one display to the
other when it runs off the edge of the screen. Keyboard focus also
moves between displays in the way you'd expect, and the X selection
propagates around. At least one of the displays involved (specifically,
the one being controlled remotely) must support the XTEST extension.
(The servers shipped by Debian support this.)
x2x is useful when you have two displays on the same desk, as you can use
a single keyboard and mouse to control both. If you want to propagate
your selections between displays which are not sharing a desk, you might
be more interested in the package propsel. Propsel does not link the
displays' keyboard and mouse, which would be confusing in some cases,
and can propagate between up to 8 displays, whereas x2x can handle only
two at a time.