This behavior is not supported in the current edition of the DockingManager. This feature (together with more extensions and improvements) has been added to our new RadDock
component. The first beta preview of the new RadDock version will be introduced later this week, so stay tuned for more information. If you would like to be explicitly notified for the new version, please write back and I will add you to the beta testers list.
the Telerik team