xdmx does slightly different thing, it allows one to build X display that uses another X displays as it's screens (everything goes through the xmdx server and l

xdmx does slightly different thing, it allows one to build X display that uses another X displays as it's screens (everything goes through the xmdx server and last time I tried it it was incredibly slow, as in significantly slower than using normal X11 over the same network). X11-only equivalent of synergy is for example x2x (https://github.com/dottedmag/x2x https://github.com/dottedmag/x2x).