Multiple Screens in Xvfb

Hi

I've been using Xvfb for a while now to do screen captures. I use the
following commands:

Xvfb :0 -screen 0 1024x768x16 -ac &
DISPLAY=:0.0 mozilla file:///srv/apps/siteimage/maximise.htm &
DISPLAY=:0.0 mozilla -remote "openURL(http://www.domain.com)" &
xwd -display 0:0 -root -out tmpout
convert -size 1024x768 -thumbnail 1024x768 tmpout image1.jpg

And it's worked great for me. The problem has come when trying to
scale. I need to have multiple apps doing this kind of thing at the
same time. I've tried to open multiple screens with these commands:

Xvfb -screen 0 800x600x16 -screen 1 800x600x16 -ac :1 &
DISPLAY=:1.0 mozilla file:///srv/apps/siteimage/maximise.htm &
DISPLAY=:1.1 mozilla file:///srv/apps/siteimage/maximise.htm &

DISPLAY=:1.0 mozilla -remote "openURL(http://www.domain1.com)" &
DISPLAY=:1.1 mozilla -remote "openURL(http://www.domain2.com)" &

But I just get a "Error: No running window found." error when I try
and browse to domain2 on screen 1.

Has anyone got any ideas? A different approach to achieve the same
thing is doable

Thanks