It is expected that the child element is limited in the parent's bounds. To override this behavior you should open new RadWindows from the browser window. The Modal
property creates an overlay background that disables its parent, it does not let it leave these bounds.
A shorter explanation is available in the documentation: http://www.telerik.com/help/aspnet-ajax/window-programming-opening-from-within-dialog.html
If the new window opens below the first one you might need to call its setActive()
method after you open it.
the Telerik team