how to show a usercontrol on MainWindow from another UserControl

I have two Usercontrol (CreationNouveauCas_uc and uc_TraiteTravail)and obviously one MainWindow and a grid inside it

<Grid x:Name="CreationNouveauCasGrid" HorizontalAlignment="Left" Height="653" Width="1170" Margin="137,30,0,0" VerticalAlignment="Top" />

I display the usercontrol CreationNouveauCas_uc on the MainWindow's Grid like this:

Private Sub btnShow_Click(sender As Object, e As RoutedEventArgs)
        Dim cnc As CreationNouveauCas_uc = New CreationNouveauCas_uc
        CreationNouveauCasGrid.Children.Clear()
        CreationNouveauCasGrid.Children.Add(cnc)
End Sub

so, I'm on the first UserControl (CreationNouveauCas_uc), and from it, I want to show the second usercontrol (uc_TT) on the same MainWindow's GridCreationNouveauCasGrid, So I do something like this

Private Sub btnShow_ucTT_Click(sender As Object, e As RoutedEventArgs)


  Dim mw As MainWindow = New MainWindow
  Dim uc_TT As ucTraiteTravail = New ucTraiteTravail                                        
  mw.CreationNouveauCasGrid.Children.Clear()                           
  mw.CreationNouveauCasGrid.Children.Add(uc_TT)
End Sub

And as I expected, it doesn't show anything.

but when I create a button on the MainWindow.xaml (it's to show the second userControl from the MainWindow) and apply this code-behind, it shows the userControl.

Private Sub btnShow_UserControl2_Click(sender As Object, e As RoutedEventArgs)
        Dim uc_TT As ucTraiteTravail = New ucTraiteTravail
        CreationNouveauCasGrid.Children.Clear()
        CreationNouveauCasGrid.Children.Add(uc_TT)
End Sub

so, here is the issue, how can I show the second user control on to the MainWindow's grid form the first usercontrol.

Any bit of possible solution is welcome