TwinView not to support xinerama?
Apparently NV's dual head solution approach differs from other vendor's one. E.g. X will recognize a Matrox G550 as two physical boards each connected to a physical display.
NV's TwinView seems to do the thing in a somewhat strange manner on its own somehow in the background. In my X config there's one FX5500 and one 1280x1024 display but in fact I successfully run two displays.
The problem: X is not aware of the second monitor, i.e. KDE does not offer multi-head options such as maximizing windows only on one display, center windows etc. And, a real problem, aspect ratios are calculated faulty. The output of tvtime, xine, mplayer etc. is twice as wide. For mplayer you can workaround this by -aspect 2:3 for 4:3 or 8:9 for 19:9 respectively.