using VNC as a passive viewer under Linux (viewing the local desktop on a remote box)

[Benno Senoner]

Hi,
I use Linux and I'd like to do the following using vnc server and vncviewer.

Machine A is running a standard X11 desktop  and machine B running too a 
standard X11 desktop
but should act as a simple passive viewer (in a window, like vncviewer) 
of the screen of machine A .

I tried to launch
vncserver -geometry 1024x768 -depth 16 -alwaysshared

on machine A, open a local vncviewer session (using localhost:1.0) which 
opens a desktop in a window.

Now on machine B I run:

vncviewer machinea:1.0

and when working on the vnc desktop on the machine A I see a copy of 
that desktop on the
screen of machine B.

This is what I want but I'd like running a native desktop on machine A 
(for simplicity and performance reasons),
without opening a vnc desktop in a window on A.

Is that possible ?
I tried an other app (XMX) that works as X Protocol Multiplexor
http://www.cs.brown.edu/software/xmx/

but this too opens a new desktop in a window on the local box.
I'd like to use the native X server that is shared (read only) with box B.

I tried an old app that simply copies the data from the local display to 
a remote display called xwatchwin:
it works but it is extremely slow because it does not apply all the 
optimizations like VNC when transfering
the data from display A to display B.

Any solution to my problem ?

Thanks in advance for infos.

cheers,
Benno