Re: Twinview, nvidia-settings and headaches.
The reason why windows are stretched over two displays when you enable twinview using nvidia-settings is that the nvidia's xinerama extension is only loaded when X is started with twinview enabled. Nvidia-settings can't load the extension afterwards. I believe xorg 7.2 will allow this soon using randr 1.2 (it will require nvidia driver support)
There's no way to kill the void except for panning. You could also consider using seperate X screens, that way both screens behave as 'two X servers' and can be given their own desktop. That way they can have different resolutions.