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>