// C# part:
    class NotifyPropertyChangedImpl : INotifyPropertyChanged
    {
        protected void NotifyPropertyChanged([CallerMemberName] string name = "")
        {
            if (PropertyChanged != null)
                PropertyChanged(this, new PropertyChangedEventArgs(name));
        }

        public event PropertyChangedEventHandler PropertyChanged;
    }

    class Data : NotifyPropertyChangedImpl
    {
        private bool _isParentSelected;

        public bool IsParentSelected
        {
            get { return this._isParentSelected; }
            set
            {
                this._isParentSelected = value;
                NotifyPropertyChanged();
            }
        }

        public string Title { get; set; }
    }

    class ParentData : NotifyPropertyChangedImpl
    {
        private bool _isSelected;
        public bool IsSelected
        {
            get { return this._isSelected; }
            set
            {
                this._isSelected = value;
                NotifyPropertyChanged();
                if (First != null)
                    First.IsParentSelected = value;
                if (Second != null)
                    Second.IsParentSelected = value;
            }
        }

        public Data First { get; set; }
        public Data Second { get; set; }
    }

    class MainVM
    {
        public IEnumerable<ParentData> Hierarchical { get; set; }
        public IEnumerable<Data> Flat { get; set; }
    }

    public partial class App : Application
    {
        protected override void OnStartup(StartupEventArgs e)
        {
            base.OnStartup(e);
            var data = new[]
            {
                new ParentData()
                {
                    First = new Data() { Title = "Big" },
                    Second = new Data { Title = "Bad" }
                },
                new ParentData()
                {
                    First = new Data() { Title = "Gray" },
                    Second = new Data { Title = "Wolf" }
                }
            };

            var mainVM = new MainVM()
            {
                Hierarchical = data,
                Flat = data.SelectMany(pd => new[] { pd.First, pd.Second }).ToArray()
            };

            var mainWindow = new MainWindow() { DataContext = mainVM };
            mainWindow.Show();
        }
    }

<!-- XAML part -->

<Application x:Class="Test.App"
             xmlns="http://schemas.microsoft.com/winfx/2006/xaml/presentation"
             xmlns:x="http://schemas.microsoft.com/winfx/2006/xaml">
</Application>

<Window x:Class="Test.MainWindow"
        xmlns="http://schemas.microsoft.com/winfx/2006/xaml/presentation"
        xmlns:x="http://schemas.microsoft.com/winfx/2006/xaml"
        xmlns:local="clr-namespace:Test"
        Title="MainWindow" Height="350" Width="525">
    <Grid>
        <Grid.ColumnDefinitions>
            <ColumnDefinition/>
            <ColumnDefinition/>
        </Grid.ColumnDefinitions>

        <DataGrid ItemsSource="{Binding Hierarchical}" AutoGenerateColumns="False">
            <DataGrid.ItemContainerStyle>
                <Style TargetType="DataGridRow">
                    <Setter Property="IsSelected" Value="{Binding IsSelected}"/>
                </Style>
            </DataGrid.ItemContainerStyle>
            <DataGrid.Columns>
                <DataGridTemplateColumn Header="First">
                    <DataGridTemplateColumn.CellTemplate>
                        <DataTemplate DataType="{x:Type local:ParentData}">
                            <TextBlock Text="{Binding First.Title}" Name="tb"/>
                            <DataTemplate.Triggers>
                                <DataTrigger Binding="{Binding IsSelected}" Value="True">
                                    <Setter Property="Background" TargetName="tb" Value="Yellow"/>
                                </DataTrigger>
                            </DataTemplate.Triggers>
                        </DataTemplate>
                    </DataGridTemplateColumn.CellTemplate>
                </DataGridTemplateColumn>
                <DataGridTemplateColumn Header="Second">
                    <DataGridTemplateColumn.CellTemplate>
                        <DataTemplate DataType="{x:Type local:ParentData}">
                            <TextBlock Text="{Binding Second.Title}" Name="tb"/>
                            <DataTemplate.Triggers>
                                <DataTrigger Binding="{Binding IsSelected}" Value="True">
                                    <Setter Property="Background" TargetName="tb" Value="Green"/>
                                </DataTrigger>
                            </DataTemplate.Triggers>
                        </DataTemplate>
                    </DataGridTemplateColumn.CellTemplate>
                </DataGridTemplateColumn>
            </DataGrid.Columns>
        </DataGrid>

        <ItemsControl ItemsSource="{Binding Flat}" Grid.Column="1">
            <ItemsControl.ItemTemplate>
                <DataTemplate DataType="{x:Type local:Data}">
                    <TextBlock Text="{Binding Title}" Name="tb"/>
                    <DataTemplate.Triggers>
                        <DataTrigger Binding="{Binding IsParentSelected}" Value="True">
                            <Setter Property="Background" TargetName="tb" Value="Blue"/>
                        </DataTrigger>
                    </DataTemplate.Triggers>
                </DataTemplate>
            </ItemsControl.ItemTemplate>
        </ItemsControl>
    </Grid>
</Window>