Re: Nvidia vs. nouveau
Uhm, you need to make sure Xinerama info is enabled, not disabled. Also, your window manager needs to support Xinerama. It's no different to Xrandr, which works the same way. (i.e. uses a single merged framebuffer and exports Xinerama information to let the window manager know how to place windows)
It is also possible to use real Xinerama or separate X screens, but these configuration types don't allow dynamic reconfiguration (AFAIK) and have more restrictions than Xrandr/TwinView.