in section device put:

Option “Twinview” “true” //enables dual monitor setup
Option “TwinViewOrientation” “Clone” //explains itself
Option “TwinViewOrientation” “LeftOf” //explains itself

further handy options are:
Option “NoLogo” “true” //no nvidia startup logo
Option “AllowGLXWithComposite” “true” //enables compiz(-fusion)

to further enable composite or compiz(-fusion) add:

Section “Extensions”
Option “Composite” “true”
EndSection

to your file

best is to add another monitor section (=your second monitor) and add it to your device section like, first put add this to your file:

Section “Monitor”
Identifier “Monitor1″
VendorName “Monitor Vendor”
ModelName “Monitor Model”
EndSection

and in your screen section add:
Monitor “Monitor1″

if you have any more questions, feel free to ask