Это то, что вы ищете?
<TextBox Height="23" HorizontalAlignment="Left" Margin="190,81,0,0" Name="TextBoxName" VerticalAlignment="Top" Width="{Binding ElementName=TextBoxPhone, Path=Width, Mode=OneWay}" />
<TextBox Height="23" HorizontalAlignment="Left" Margin="194,192,0,0" Name="TextBoxPhone" VerticalAlignment="Top" Width="120" />