I have a machine that runs X but doesn't listen to inet connections (via -nolisten-tcp). What if I want to use x2x to this machine? Simple.

 ssh -tY thatmachine 'x2x -from $DISPLAY -to :0 -west' 

This will ssh to 'thatmachine' and forward X. This set's $DISPLAY on the remote machine, which you can then invoke x2x with specifying 'from' as $DISPLAY. This has the same effect as invoking:

 x2x -to thatmachine:0 -west 

Except you don't need to allow tcp X connections. :)