I recently talked about Cross Platform Development Options in Windows 8 and Windows Phone 8
In this blog post I describe how you can achieve Cross-Platform development across Windows 8 and Windows Phone 8 specifically using C# + XAML. We will be developing a simple Calculator App for both the platforms with common library and minimum platform specific layer. The key is to use "Portable Class Library" template.
Windows 8 App
Windows Phone 8 App
The steps that we are going to do this in section is EXACTLY same as the steps we did in the previous sections.
As you can see, using the Portable Class Library, we were able to pull out most of the code into a common library, which we then used as reference in the Windows 8 and Windows Phone 8 App. The only thing that we had to do in the Windows 8 and Windows Phone 8 App was defining the User Interface and wiring the User Interface with the common library. Note that the UI layer was exactly same for both platforms. With some effort, even this code can be pulled out in the common library.