There are a couple of things that look wrong to me. First off the Twinview options should go under "Device" not "Screen". Next "TwinView" does not have a "On" option. Finally MetaModes are a single array of pairs. So the Twinview options should be moved to Device and the device should look something like this.
Code:
Section "Device"
Identifier "Nvidia"
Driver "nvidia"
Option "Twinview"
Option "SecondMonitorHorizSync" "30.0-97.0"
Option "SecondMonitorVertRefresh" "50-180"
Option "MetaModes" "1600x1200, 1600x1200; 1152x864, 1152x864; 1024x768,1024x768"
Option "TwinViewOrientation" "RightOf"
EndSection
Identifier should be whatever you named it (not necessarily Nvidia) but Deiver and the rest look right for what you gave. You can also have different resolution settings for each monitor, I kept them the same for simplicity.