Currently this is not possible out-of-the-box. In order to keep the other windows on top of the modal RadWindow you need to somehow activate them every time the Modal window is activated which will not let the user to use the modal window.
Currently we are working on an update in the RadWindow control and we will add IsTopmost
property which will let you make the child windows topmost and they will always stay over the modal window. This update will be release with 2011 Q2 version of RadControls.
Hope this helps.
the Telerik team
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