In order to achieve the desired you would need to manually manage a collection containing the the Headers and IsTopmost properties of all opened Windows. Whenever a new Window is opened you have to add it to the collection and whenever the Window is closed to removed from there. For mode details regarding data binding of RadMenu you can check the following article from our online help documentation:
For your convenience I have prepared a project demonstrating a sample solution of the desired.
Hope this helps.
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