Nvidia GPU's and single large display mode

Is there any way to force a single Nvida GPU (gtx 285 or gtx 5xx) to use 2 monitors as a single large display or do I have to use SLI. Even with SLI is it possible to use just 2 monitors or do you NEED 3?
 
what happens then is that one monitor is set as primary and the other secondary. which means that 3d games will still only run on one monitor. (the primary) In SLD mode windows will think that the monitor is a single unit at 3840x1080 if you have 2 1080p monitors.