Multiple LayoutControl instances with LayoutControlToolBox share single ToolBoxView instance.
This makes the ToolBoxView useless because it indicates wrong structure of foreign LayoutControl.
Workaround is to instantiate the ToolBoxViews manually:
<telerik:RadLayoutControl.AdditionalCanvasItems>
<telerik:LayoutControlToolBox LayoutControl="{Binding ElementName=layoutControl4}" >
<telerik:LayoutControlToolBox.ToolBoxView>
<telerik:LayoutControlToolBoxView />
</telerik:LayoutControlToolBox.ToolBoxView>
</telerik:LayoutControlToolBox>
</telerik:RadLayoutControl.AdditionalCanvasItems>
By default they are instantiated internally.