I think in your
VendorName "NVIDIA Corporation"
bit...the "Screen 2" shouldn't be there... As far as I understand that Screen 0,1 is to specify which output of a dual head card to use and since this is the first port on this card to use either set it to 0 or take it out completely. Just an idea...I'm no expert since I'm having similar troubles...have a dual head 6800U which I can run both at the same time as independant screens but running one at a time (trying to use a separate kbd and mouse on each) it gives me issues (same as yours) for the second monitor.
I think this is due to it only using one device section (the one with "Screen 2") and not knowing/understanding whats going on? Lol can someone help me too?