How might I implement something like the VS XAML designer?

Are there any samples that mirror the XAML designer in Visual Studio? Or, more generally, what would be the best way to implement a document tab that has a top half using one control and the bottom half using another control, with a custom separator between them that could be used for resizing either half and potentially other things.

Looking through the documentation, it seems like nested dock sites might be the way to go, except that I don't see a way to provide a custom separator with re-templating, and I don't want the dock sites to be removable from the document tab.

Any thoughts on the best approach for this are appreciated.

Thanks,

-Craig

[Modified at 11/03/2011 05:31 PM]