Arguably they're already "merged" from a technical standpoint, so all that's left is the UI - and I don't see how you can really translate one to the other.
It makes even less sense than forcing Metro on the desktop.
The Start Page for both is basically a block of resizable squares. It would be different code but I don't see why it couldn't just be Windows ... Mobile (gulp) or touch or something with all the development being under the same umbrella.