multibinding in WP8 using MVVMLight

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Windows-Phone-8

Hello,
I am develeoping WP8 app using MVVMLight. I have 4 radio buttons and one textbox. I want textbox should be visible for first two radio buttons and for rest of the two buttons it should not be visible.How should I do that in MVVMLight?

4 radio buttons

<RadioButton x:Name="rbcoldcall" Content="Prospecting (New)" BorderBrush="Black" Grid.Column="0" Grid.Row="2" Foreground="Black" Margin="0,0,0,10" HorizontalAlignment="Right" Width="205" Command="{Binding RadioBtnCommand, Mode=TwoWay}" CommandParameter="{Binding Content, ElementName=rbcoldcall}" IsChecked="false"  ></RadioButton>
                                        <!--IsChecked="{Binding AllowEdit,Mode=TwoWay, Source={StaticResource BtoVViewModel}}"-->
                                        <RadioButton x:Name="rbcoldcallf" Content="Prospecting (Follow Up)" BorderBrush="Black" HorizontalAlignment="Left" Grid.Column="1" Grid.Row="2" Foreground="Black" Width="269"  Margin="0,0,0,10" Command="{Binding RadioBtnCommand, Mode=OneWay}" CommandParameter="{Binding Content, ElementName=rbcoldcallf}" IsChecked="false"  />
                                        <RadioButton x:Name="rbenquiry" BorderBrush="Black" Content="Lead (New)" HorizontalAlignment="Left" Grid.Column="0" Grid.Row="3" Foreground="Black" Width="205" Margin="0,0,0,10" Command="{Binding RadioBtnCommand, Mode=OneWay}" CommandParameter="{Binding Content, ElementName=rbenquiry}" IsChecked="false" />
                                        <RadioButton x:Name="rbenquiryf" BorderBrush="Black" Content="Lead (Follow Up)" HorizontalAlignment="Left" Grid.Column="1" Grid.Row="3" Foreground="Black" Width="255" Command="{Binding RadioBtnCommand, Mode=OneWay}" CommandParameter="{Binding Content, RelativeSource={RelativeSource Self}}" IsChecked="false" />
                                        <RadioButton x:Name="rbother" Content="Other" BorderBrush="Black" HorizontalAlignment="Left" Grid.Column="0" Grid.Row="4" Foreground="Black" Width="187" Command="{Binding RadioBtnCommand, Mode=OneWay}" CommandParameter="{Binding Content, ElementName=rbother}" IsChecked="false"/>

one txt box and one list picker

 <StackPanel>

 <TextBox Text="" x:Name="txtCasetitle" TextWrapping="Wrap" AcceptsReturn="True" Foreground="Black" Visibility="{Binding Path=IsChecked, Converter={StaticResource BtoVConverter}, Mode=TwoWay,ElementName=rbcoldcall}" />
                                
<toolkit:ListPicker x:Name="lpCaseTitle" Margin="12,36,10,10" Foreground="Black"  ItemsSource="{Binding ObVisitType, Mode=TwoWay}" SelectedItem="{Binding SelectedVisitType, Mode=TwoWay}" Visibility="{Binding Path=IsChecked, Converter={StaticResource BtoVConverter}, Mode=TwoWay,ElementName=rbcoldcallf}">
                                    </toolkit:ListPicker>
                                </StackPanel>

If My first and third radio button clicked then I want to Visible textbox
If second and forth radio button clicked then I want to Visible ListPicker

I have written the boolean to visible Converter

public class BooleanToVisibilityConverter:IValueConverter
{

    public object Convert(object value, Type targetType, object parameter, System.Globalization.CultureInfo culture)
    {
        if (value != null)
        {
            bool isVisible = (bool)value;
            return isVisible ? Visibility.Visible : Visibility.Collapsed;
        }
        return null;

    }

    public object ConvertBack(object value, Type targetType, object parameter, System.Globalization.CultureInfo culture)
    {
        return (Visibility)value == Visibility.Visible;
    }
}