AFACIS from testing the command

   X :1 -listen tcp vt8

does not open a tcp port anymore.  This is what

   netstat -tuwp

reports here. Side effect is that I'm not able to connect such an X server with

  xterm -display <host>:1

nor with

  Xdmx :1 -display :0 -display <host>:1 +xinerama

instead I see

  No protocol specified
  xterm: Xt error: Can't open display: <host>:1

  No protocol specified
  (Fatal Error) dmx: dmxOpenDisplay: Unable to open display <host>:1

with <host> equal to the FQDN of my second host here.

IMHO it is OK is -nolisten is the default but this look more like the code for
listen on tcp has been disabled on compile time or removed from source by
default.