How to use LinkDockSite for nested DockSite

Hello,

How to use LinkDockSite for nested DockSite?

Source code is below.

MainWindow.xaml

<Window
    x:Class="LinkedDockSiteSample.MainWindow"
    xmlns="http://schemas.microsoft.com/winfx/2006/xaml/presentation"
    xmlns:x="http://schemas.microsoft.com/winfx/2006/xaml"
    xmlns:d="http://schemas.microsoft.com/expression/blend/2008"
    xmlns:docking="http://schemas.actiprosoftware.com/winfx/xaml/docking"
    xmlns:local="clr-namespace:LinkedDockSiteSample"
    xmlns:mc="http://schemas.openxmlformats.org/markup-compatibility/2006"
    Title="MainWindow"
    mc:Ignorable="d">
    <Grid>
        <!--  DockSite1  -->
        <docking:DockSite x:Name="DockSite1">
            <docking:SplitContainer>
                <docking:SplitContainer>
                    <docking:ToolWindowContainer>
                        <docking:ToolWindow Title="Tool Window 1" />
                        <docking:ToolWindow Title="Tool Window 2" />
                    </docking:ToolWindowContainer>
                </docking:SplitContainer>

                <docking:Workspace>
                    <docking:ToolWindowContainer>
                        <docking:ToolWindow
                            Title="Fixed"
                            CanDragTab="False"
                            HasTitleBar="False">

                            <!--  DockSite2  -->
                            <docking:DockSite x:Name="DockSite2" IsHitTestVisible="True">
                                <docking:Workspace>
                                    <docking:TabbedMdiHost>
                                        <docking:TabbedMdiContainer>
                                            <docking:DocumentWindow Title="Document1.txt">
                                                <TextBox
                                                    BorderThickness="0"
                                                    Text="Move other tool windows around the 'fixed' tool window."
                                                    TextWrapping="Wrap" />
                                            </docking:DocumentWindow>
                                        </docking:TabbedMdiContainer>
                                    </docking:TabbedMdiHost>
                                </docking:Workspace>
                            </docking:DockSite>
                        </docking:ToolWindow>
                    </docking:ToolWindowContainer>
                </docking:Workspace>
            </docking:SplitContainer>
        </docking:DockSite>
    </Grid>
</Window>

MainWindow.xaml.cs

    public partial class MainWindow : Window
    {
        public MainWindow()
        {
            InitializeComponent();

            DockSite1.LinkDockSite(DockSite2);
        }
    }