I've got several apps in the store. Most of the UI code is fully shared, and moderately adoptive to screen size. In a few places, I needed something special for one or the other.
My trick is that the 8.1 universal apps have two mainpage.xaml files (one for desktop, one for phone). I just make a shared UserControl. Each MainPage just has one object, which is the shared control
(BTW: I work at Microsoft, but not in the group that does XAML; my way works but that doesn't mean there isn't a better way)