I've noticed you have opened a support ticket with the same question but for Silverlight. However the approach we have provided you there could be used for WPF as well. I'm copying our answer here in order to be available to the community:
In order to dock a specific RadPane instance using the code behind you could do the following:
- Create a new RadPaneGroup
- Use the RemoveFromParent method on the RadPane which is being docked
- Create a new RadSplitContainer and set its InitialPosition to the desired DockState value
- Add the RadPane that is being docked to the Items of the RadPaneGroup
- Add the RadPaneGroup to the new RadSplitContainer
- Finally add the RadSplitContainer to the RadDocking Items.
I'm also attaching a sample project for WPF which demonstrates the same approach, note that the example also demonstrates how to set the size of the SplitContainer when creating it (as you have also asked about that in the ticket).
Hope this will work for you.
Check out the Telerik Platform - the only platform that combines a rich set of UI tools with powerful cloud services to develop web, hybrid and native mobile apps.