Yes, adding such functionality is in our plans for RadWindow, and it will be available during this year, however we cannot tell exactly when. Until this functionality is released, however, you may use RadDock or RadToolTip, as they load content that belongs to the same page, and the interaction between controls is much straightforward and easy.
the Telerik team
Check out Telerik Trainer
, the state of the art learning tool for Telerik products.