Rather than using the VBA code listed in the "Advanced Custom Mapping" section, you might want to read a little further on and go to the 'Content Control Tools' page and download the addin there.
A simpler alternative would be to use a ContentControlOnExit macro in the 'ThisDocument' code module of either the document or its template, coded along the lines of:
Code:
Private Sub Document_ContentControlOnExit(ByVal CCtrl As ContentControl, Cancel As Boolean)
If CCtrl.Title <> "Parent" Then Exit Sub
With CCtrl
If .ShowingPlaceholderText = False Then
ActiveDocument.SelectContentControlsByTitle("Child")(1).Range.Text = .Range.Text
Else
ActiveDocument.SelectContentControlsByTitle("Child")(1).Range.Text = "XXXX"
End If
End With
End Sub
where 'Parent' is the title of the Content Control you type in and 'Child' is the title of the Content Control the typed string is to be replicated in.