Alright, well i'm a tech head so obviously vista is my choice of OS for multiple reasons, so yeah.
I'll give it a shot at using -1 or disabled for the monitor that's missing, but under vista you cannot use a 'horizontal span' mode for the desktop, you are limited to; Single (one monitor), Clone (second monitor is clone of the first) and Dual View (configured independently), so while you can expand your desktop onto another monitor like an attachment you can't use both monitors as a single desktop.
Both my computers have individual setups, where one has 3x cards; GPU+GPU+PPU and the other has 2x cards; GPU+PPU. The one i'm using atm is GPU+PPU so only 1 card is being connected to monitors where the other one is being used as a physics processor, so I don't have to worry about
and honestly it should work much faster with a driver-enabled solution rather than one that looks to offload frames to another card before outputting them.