To move the header of the RadTabbedWindow to a separate title bar, you need to edit the control template of the TabbedWindowTabControl element.
I've gone ahead and prepared a small sample project which demonstrates how to achieve this in the Office_Black theme. What I've done is to introduce two row definitions in the PART_HeaderGrid element and distribute its children along these definitions. Please note that as the templates differ from one theme to another, you will need to extract and edit the control's template accordingly if you're using another theme.
Please have a look and let me know if you find this helpful.
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