But that's what didn't work with Windows 8. For majority of consumers UI is operating system. Hence the confusion with difference between Windows RT and Windows 8. Hence the perception that Windows 8 has two isolated operating systems in it.
I think what he means is more like iOS and OSX. Both run a very similar operating system but completely different UI. The best approach would be to have a common core with Windows and Metro interfaces, but separate them. Desktops and Laptops run Windows UI. Cellphones, Tablets, and TVs run Metro UI. This way you can maintain backwards compatibility on Windows side but allow the Metro side to be all new.
