The Dual Monitor setup is defined by in the xorg.conf file "Monitors" 
section, you need 2 entries.
http://www.redhat.com/docs/manuals/enterprise/RHEL-4-Manual/ref-guide/s1-x-clients.html
http://www.redhat.com/docs/manuals/enterprise/RHEL-4-Manual/sysadmin-guide/s1-xconfig-dualhead.html
http://www.redhat.com/magazine/014dec05/features/multihead/

The Gentoo wiki is rather helpfull too...
http://gentoo-wiki.com/HOWTO_Dual_Monitors
http://darrylclarke.com/Xorg_Conf.html

Spanning mode is called Xinerama
http://tldp.org/HOWTO/Xinerama-HOWTO/

AnandTech: Matrox Millennium G450 Under Linux
http://www.anandtech.com/showdoc.html?i=1322&p=3

The NWS & Comet developed a training workstation with dual head & 
documented it awhile back, it's looking dated now, but may prove 
useful... http://strc.comet.ucar.edu/wes/documents/dual_head.htm

-HTH Art@JAX

John Haggerty wrote:
> It looks like my real problem is going to be the dual monitors in SL3.
>
> I guess the guidance I need is how to configure the dual head graphics 
> cards; what do you have to do to the Matrox cards, for example, to get 
> the dual heads working?  This is a PCIe motherboard, so it looks like 
> I'd want the P650 if I need to trade in my PNY cards, since that's a 
> PCIE x16 dual DVI jobby.  But if I get the driver from the Matrox site 
> (http://www.matrox.com/mga/support/drivers/latest/home.cfm) what do I 
> have to do to get the second head working?  I can't see it in the SL3 
> docs, and SL4 has display configuration for the second monitor.  The 
> nvidia driver actually works for the first head with the PNY card; 
> it's just not clear what's needed to get the second half working.
>
> Thanks for the information--this thread has already cleared up some 
> questions for me.
>
> Miles O'Neal wrote:
>
>> For basic dual head, if you don't need serious speed or 3D,
>> the Matrox cards are wonderful.  We're still running a bunch
>> of G450s.  (If anyone can point me to performance comparisons
>> of the G550 and P650 vs the G450, I would appreciate it; there
>> doesn't seem to be much on the web in this vein!)  We use the
>> Matrox drivers, and everything works beautifully, albeit slowly
>> (esp. with GL).
>>
>> The only nVidia dual head we've tried is a Jaton MX4000Twin.
>> The nv driver works very well for most things, but we needed
>> GL for one of the Cadence tools, and ended up having to install
>> the nvidia driver from their site.  It had some other issues.
>> These work fine for us in single head mode. 8^/
>>
>> We've also tried an Asus 9250 (ATI) with one RGB and one DVI
>> out (one CRT, one flatpanel LCD).  Again, we had to load ATI's
>> proprietary driver.  It mostly works, but again 3D bit us;
>> the GL support only extends up to a certain screen size, which
>> is about 2/3 to 3/4 of the way across the second monitor.  If
>> any part of a window is past that point, no GL works within
>> that window.
>>
>> -Miles
>

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