Screenproperties >Properties > Advanced > Geforce xxx
(I probably have an old forceWare version.)
However, I give it a try.
If clone or dual view mode is selected in NVIDIA nView Properties there is a choice in the classic (=old style) control panel (which I still use) between "Video Overlay Settings" and "Tools", i.e. "Full Screen Video". If you click on that, an option appears to select "Full screen device". What options can you select there and what happens if you change?
P.S.:
(1) Probably the new style interface control panel has similar options, but I don't use it.
(2) Isn't clone mode a more appropriate option for you?