Thank you for contacting us.
I would suggest you setting the RadWindow.Owner property to the parent RadWindow. In this case, the child RadWindow knows which RadWindow is his parent and it won't go behind it. You could refer to our online documentation - http://www.telerik.com/help/silverlight/radwindow-features-positioning.html.
Anyway, if you need to get all opened RadWindows and manage them manually, the RadWindowManager could be very helpful.
I hope this helps.
the Telerik team
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