I am not quite sure what you mean by "on top of each other" - do you mean that the RadPanes look like nested one in another? If it is so, please open a new support ticket and send us more information about this - test project, information about your VS.NET version, screenshots, etc.
The correct look of the RadPanes in design mode is the one I got when I tested your code - you can find it in the attached .jpeg file. This appearance is the one which we found for most suitable after a research and we believe that it will suite you, too.
the Telerik team