Achieving the desired behavior depends if or not there is an DocumentHost in the RadDocking control. If there is no DocumentHost in the control and the HasDocumentHost property is set to False, simply adding the SplitContainer to the Items of the control will make that Pane centered and stretched. If the HasDocumentHost i set to True (its default value) you will need to add the newly created Pane to the SplitContainer that is placed inside the DocumentHost.
I recorded a short video showing how to achieve both described scenarios, hope this is helpful. You can also find the sample project attached to my response.
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