HOWTO set size of viewport/screen in dual headed computer.

I have a computer with two(2) PCI video cards, an S3Virge(256M) and a Savage4(128M), both of which work fine to run a monitor.

I am now attempting to have them work together after failing with the S3virge and an Alliance AT3D.

X is Xorg.

Running
X -configuration, then
X -config /root/xorg.conf.new

gives dual head and the mouse cursor moves between them nicely and smoothly. Based on mouse cursor, the resolutaion is probably 640x480 for each.

However, when I graft the details into my working /etc/X11/xorg.conf, I get an error complaining that the Savage 4 has insufficent Videoram available for 3D - try a lower color depth or smaller desktop.

My guess is that it is seeing the S3Virge first (PCI:0:12:0) and its 256Mb with a NEC V500 screen and then trying to impose the 1600x??? resolution that it is capable of on the the Savage4 driving a NEC 5FGp screen (max 1280?).

so how and where do I specify desktop size?

All the configuration information I have found todate has not mentioned this.

T.i.A.