All you need is one XAML and two text boxes. So here goes XAML:
<Window x:Class="WpfApplication3.MainWindow" xmlns="http://schemas.microsoft.com/winfx/2006/xaml/presentation" xmlns:x="http://schemas.microsoft.com/winfx/2006/xaml" Title="MainWindow" Height="350" Width="525"> <Grid> <TextBox HorizontalAlignment="Left" Height="23" Margin="96,91,0,0" TextWrapping="Wrap" Text="{Binding Path=Text, ElementName=myTest}" VerticalAlignment="Top" Width="120"/> <TextBox HorizontalAlignment="Left" Height="23" Margin="120,200,0,0" TextWrapping="Wrap" Text="TextBox" VerticalAlignment="Top" Width="120" x:Name="myTest"/> </Grid> </Window>