If you have a sufficiently modern Linux installation, and reasonably new nvidia cards and driver (I have 185 at work(ubuntu), 190 at home(Fedora12)), you've already done the hard part.
All you need to do now is install the nvidia-settings package (yum install nvidia-settings), reboot and plug in the new card and monitor.
Then run nvidia-settings as root.
Both your GPUs will (should) be listed along with the monitors attached to each.
Under "X Server Display Configuration" you'll see all three monitors, though one may be disabled. You'll be able enable it on the configuration line by selecting that screen first. You can also place the screens where you want (left, right, top, bottom ... whatever) here as well.
Note: so far as I know, with two separate cards you will have to either use Xinerama or separate x displays. (it's possible that in an SLI setup you'll be able to run more than two monitors in twin-view. I'm not sure about that setup as I've never used it. I ran two cards with no SLI).
Xinerama will not allow for a compositing X display (no fancy 3d window effects) as well as any other issues you may have with Xinerama.
You shouldn't have to edit your xorg.conf directly any more.
I should advise you not to expect alot. I recently got to the end of my tether and ended up buying a new larger monitor to replace 2 smaller ones to go from 3 to 2 monitors because linux triple-head and multi-gpu hardware acceleration is nothing short of rubbish. Dual head is good times, Triple was just faf, pain, and annoyances. I got 3 monitors working in 2d, but no xrandr meant everything from googleearth to my mp3 player's visualisations (and everything in-between, particularly most games) just crashed out., or my personal faviorite - would crash X losing all your unsaved work. – Sirex – 2011-02-09T13:42:46.780
Try that one, it's for mandriva, but should be working.
– Bobby – 2010-01-20T12:57:20.827