wpf Datagrid Checkbox NotifyOnTargetUpdated not works

My Class:

    public class BarCode : INotifyPropertyChanged
{
    public event PropertyChangedEventHandler PropertyChanged;
    protected virtual void OnPropertyChanged(string propertyName)
    {
        if (PropertyChanged != null)
            PropertyChanged(this, new PropertyChangedEventArgs(propertyName));
    }

    public string code { get; set; }
    public string name { get; set; }
    public Visibility special1 { get; set; }
    public Visibility special2 { get; set; }
    private bool _x1, _x2;
    public bool x1
    {
        get
        {
            return _x1;
        }
        set
        {
            if (value && _x2)
                _x2 = false;
            _x1 = value;
            OnPropertyChanged("X1");
        }
    }
    public bool x2
    {
        get
        {
            return _x2;
        }
        set
        {
            if (value && _x1)
                _x1 = false;
            _x2 = value;
            OnPropertyChanged("X2");
        }
    }
}

XAML:

                <DataGridTemplateColumn Header="X">
                <DataGridTemplateColumn.CellStyle>
                    <Style TargetType="DataGridCell">
                        <Setter Property="Visibility" Value="{Binding special1}"></Setter>
                    </Style>
                </DataGridTemplateColumn.CellStyle>
                <DataGridTemplateColumn.CellTemplate>
                    <DataTemplate>
                        <CheckBox IsChecked="{Binding Path=.x1, UpdateSourceTrigger=PropertyChanged, NotifyOnTargetUpdated=True}"/>
                    </DataTemplate>
                </DataGridTemplateColumn.CellTemplate>
            </DataGridTemplateColumn>
            <DataGridTemplateColumn Header="X">
                <DataGridTemplateColumn.CellStyle>
                    <Style TargetType="DataGridCell">
                        <Setter Property="Visibility" Value="{Binding special2}"></Setter>
                    </Style>
                </DataGridTemplateColumn.CellStyle>
                <DataGridTemplateColumn.CellTemplate>
                    <DataTemplate>
                        <CheckBox IsChecked="{Binding Path=.x2, UpdateSourceTrigger=PropertyChanged, NotifyOnTargetUpdated=True}"/>
                    </DataTemplate>
                </DataGridTemplateColumn.CellTemplate>
            </DataGridTemplateColumn>

I have two Checkboxes. When the first is checked and i want to check the second one, then the first one shall dischecking. But it will not upadeting. The check mark leaves only when im scrolling out of the visibility.