With Xfree86 4.x you can use Xinerama extension, "which allows applications
and window managers to use the two (or more) physical displays as one large
virtual display" (from Xinerama - How To). For that you can either edit you
XF86Config and make the necessary changes for your video cards and monitors,
or you can use the Matrox driver together with Powerdesk (you can download it
from Matrox site, under Linux drivers).
The Xinerama - How To has some examples of XF86Configs, so you can change
them to suit your needs. On Matrox site, there are also detailed
instructions on how to install both the driver and Powerdesk.
Hope it helps