Windows 8 is expected to run across a variety of formats, desktops, tablets, netbooks, laptops, x86 and ARM. So the question is can Windows 8 and XBox 720 be merged so that Windows 8 will also run on Xbox720?

Here is the issue. Consider that when Xbox developers write code for their games, they specifically target circuits, like vector units, and shaders that exist in an Xbox 360 unit. However, those elements may not exist on a PC. You can have emulators in place to make the transition. Yet, the Xbox user interface is distinctively different from the one on a PC. Xbox users may not be enamored by a less than optimum interface for their games.
Windows 8 and XBox 720 may be more similar than you thought