It took me a while to understand the question, and then another while to transform the coe from VB to C# (as I am a C# addict) and then the solution back to VB, but I think the answer lies in some TextChanged event handlers. The code below fills all boxes.

    Private Sub Text1_Changed(ByVal sender As Object, ByVal e As System.Windows.RoutedEventArgs) Handles TextBlock1.TextChanged
        TextBlock3.Text = TextBlock1.Text
    End Sub

    Private Sub Text2_Changed(ByVal sender As Object, ByVal e As System.Windows.RoutedEventArgs) Handles TextBlock2.TextChanged
        TextBlock4.Text = TextBlock2.Text
    End Sub

    Private Sub Text5_Changed(ByVal sender As Object, ByVal e As System.Windows.RoutedEventArgs) Handles TextBlock5.TextChanged
        TextBlock2.Text = TextBlock5.Text
    End Sub

    Private Sub Text6_Changed(ByVal sender As Object, ByVal e As System.Windows.RoutedEventArgs) Handles TextBlock6.TextChanged
        TextBlock1.Text = TextBlock6.Text
    End Sub

The Attribute Loaded=MainWindow_Loaded is missing. Anyway, I'd rather use MVVM to fill control's values.

LKZ:

Gunter got it correct

Alternatively I guess we can also use databinding . The following snippet works for me

 <Grid>
        <WrapPanel Orientation="Vertical">
            <TextBox Name="TextBlock1" Width="100" Text="{Binding ElementName=TextBlock6, Path=Text}" />
            <TextBox Name="TextBlock2" Width="100" Text="{Binding ElementName=TextBlock5, Path=Text}" />
            <TextBox Name="TextBlock3" Width="100"  Text="{Binding ElementName=TextBlock1, Path=Text}"/>
            <TextBox Name="TextBlock4" Width="100" Text="{Binding ElementName=TextBlock2, Path=Text}" />
            <TextBox Name="TextBlock5" Width="100" />
            <TextBox Name="TextBlock6" Width="100" />
            <TextBox Name="TextBlock7" Width="100"/>
        </WrapPanel>
    </Grid>

Thank you for this mind twisting question Gaurav. Unfortunately it looks like I am a little bit too late, so the easiest were already suggested. Please see my solution below. As you can see it doesn't utilise any of VB .NET features used in the previous answers. It's not the easiest, not the most straightforward way and not the best design, however it will do the job and completely satisfy the requirements of the question:

XAML

<src:MyTextBox x:Name="TextBlock1" Width="100"/>
<src:MyTextBox x:Name="TextBlock2" Width="100"/>
<src:MyTextBox x:Name="TextBlock3" Width="100"/>
<src:MyTextBox x:Name="TextBlock4" Width="100"/>
<src:MyTextBox x:Name="TextBlock5" Width="100"/>
<src:MyTextBox x:Name="TextBlock6" Width="100"/>
<src:MyTextBox x:Name="TextBlock7" Width="100"/>

You also need to add attribute xmlns:src="clr-namespace:[Project Name]" to your Window tag

Code

Public Class MyTextBox 
Inherits TextBox Public 
dependentTextBox As MyTextBox 

Public Overloads Property Text As String 
Get 
Return MyBase.Text 
End Get 
Set(ByVal value As String) 
MyBase.Text = value 
If Not IsNothing(dependentTextBox) Then 
dependentTextBox.Text = value 
End If 
End Set 
End Property 
End Class 

Class MainWindow 
Public Sub New() 
InitializeComponent() 
TextBlock6.dependentTextBox = TextBlock1 
TextBlock5.dependentTextBox = TextBlock2 
TextBlock1.dependentTextBox = TextBlock3 
TextBlock2.dependentTextBox = TextBlock4 
End Sub

... then the rest of MainWindow class provided in the question