Я бы использовал для этого триггер стиля.
<ComboBox Name="myCombo" HorizontalAlignment="Left" Margin="10,10,0,0" VerticalAlignment="Top" Width="120" SelectedIndex="0" SelectedValuePath="Content">
<ComboBoxItem Content="Visible" HorizontalAlignment="Left" Width="117.714285714286"/>
<ComboBoxItem Content="Hidden" HorizontalAlignment="Left" Width="117.714285714286"/>
</ComboBox>
<TextBox HorizontalAlignment="Left" Height="23" Margin="10,37,0,0" TextWrapping="Wrap" Text="TextBox" VerticalAlignment="Top" Width="120">
<TextBox.Style>
<Style TargetType="TextBox">
<Style.Triggers>
<DataTrigger Binding="{Binding ElementName=myCombo, Path=SelectedValue}" Value="Hidden">
<Setter Property="Visibility" Value="Hidden"/>
</DataTrigger>
</Style.Triggers>
</Style>
</TextBox.Style>
</TextBox>