This happens automatically if you load an entire page in the first RadWindow. This creates an iframe that hosts the child page and thus all controls are confined in it and move together with the parent window. The KeepInScreenBounds property of the child window can help keep it in the visible viewport of the iframe.
When you are using the ContentTemplate, however, you have two independent elements with absolute position and they cannot be linked like this. You could move the second one with code, attached to the OnClientDragEnd event of the first one, e.g.:
var second = $find("<%=RadWindow2.ClientID %>");
var firstBounds = sender.getWindowBounds();
second.moveTo(firstBounds.x + 20, firstBounds.y + 40);
but all this will do is move the second "automatically", the end user will still be able to move, maximize, minimize and pin it. You can disable these behaviors (e.g. set the Behaviors property of the second one to None). You can test this out and if it fits your needs you can further build on top of it, but there will still be no actual relationship between the two.
the Telerik team
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