I don't believe that is possible. Most VNC programs don't even recognize when there is a dual screen involved. Also, if you think about it, even on a dual screen system, apps don't really recognize that there is a second screen. Open just about any Windows app and make it go full screen and it will not extend itself into the second screen. If a window isn't maximized you can resize it usually to extend into the second screen, but that's more a function of the OS recognizing that the second screen exists and not the app.
The only place that I've seen what you want to work is with the screen sharing capabilities on the Mac running OS X. I have a dual screen Mac Pro at work and an iMac at home and if I do a screen share to my work Mac, I get both screens on my iMac. But even then, it's all compressed into the single iMac screen (even though I have dual screens on my iMac).
