Xvfb

[Collins, Cris]

I am trying to get dual heads and xinerama to work with Xvfb and getting:
xdpyinfo -ext XINERAMA

Xlib: extension "XINERAMA" missing on display ":0.0".
XINERAMA extension not supported by server


Xvfb :90 -screen 0 1024x2048x24 +xinerama &
mwm :90&

I have also tried
Xvfb :90 -screen 0 1024x2048x24 -screen 1 1024x2048x24
and
Xvfb :90 -screen 0 1024x2048x24 -screen 1 1024x2048x24 +xinerama
The later Seg faults.

Any ideas why xinerama is not working.