This behavior is intentional. The designer of a RadSplitContainer residing in a RadDock will remove these options from the smart tag. This is done because the layout of RadDock is dynamic and controlled entirely by the docking framework and we do not want to have some empty panels explicitly created by the user.
On the other hand this behavior should not be true for split containers residing in a DockWindow instance. We will think of some smarter way to handle this scenario.
Currently, you may simply build your layouts on stand-alone user controls and drop these controls on the dock windows.
I would like to thank you for your feedback and for bringing this issue to our attention.
the Telerik team