The communication can take place via special property values; the two
applications can change the value of a property and each watch for changes to
it. If the communication is two-directional then two properties can be used.
The technique is appropriate for small messages which can be encoded easily.

It is expensive and unnecessary to communicate via properties on the root
window; a window owned by one of the applications is preferable.  There is a
problem, however, in communicating the window ID from one application to the
other.

The application owning the window can assert ownership of a specially-named
selection; the selection can be registered with the X Registry to ensure its
uniqueness. The second application loops, requesting the value of the
selection; the first application encodes the ID of its window and sends it.
The second application can then use XSelectInput() to get PropertyNotify
events on that window.

Thereafter, communication is via that window via XGetWindowProperty() and
XChangeProperty().

Watch also for the deletion of the property in order to disconnect properly;
one of the applications may have exited.