I was looking for the same i.e. to compare the contents of two shelveets without needing to unshelve one of them. End up writing a Visual Studio extension for the functionality.
The extension is available for Visual studio 2013
http://visualstudiogallery.msdn.microsoft.com/1fa59764-efc6-457a-a5f3-81ca68e280e8
and for Visual studio 2012
http://visualstudiogallery.msdn.microsoft.com/33c521d4-73fc-48be-969d-fa68c511341f
Please feel free to use and give your feedback.