<Window x:Class="Grid_In_Grid"
    xmlns="http://schemas.microsoft.com/winfx/2006/xaml/presentation"
    xmlns:x="http://schemas.microsoft.com/winfx/2006/xaml"
    Title="Grid_In_Grid" Height="321" Width="599"
    x:Name="MyWindow" DataContext="{Binding}">
    <!--<Window.Resources>
        <local:SubDataConverter x:Key="DataConverter"></local:SubDataConverter>
    </Window.Resources> -->   
 
        <Grid >
        <DataGrid AutoGenerateColumns="False" Height="246" _
          HorizontalAlignment="Stretch" ItemsSource="{Binding hauptdaten}"
                  Margin="12,12,298,24" x:Name="MyDG" _
                  VerticalAlignment="Stretch" Width="267" RowHeight="50">
            <DataGrid.Columns>
                <DataGridTextColumn Header="Spalte1" Binding="{Binding" & _
                  "Spalte1}" />
                <DataGridTemplateColumn Header="Übersicht" IsReadOnly="True">
                    <DataGridTemplateColumn.CellTemplate>
                        <DataTemplate>
                            <DataGrid AutoGenerateColumns="False" _
                              ItemsSource="{Binding unterdaten}" _
                              x:Name="MySubDG">
                                <!-- , Converter={StaticResource DataConverter} _
                                -->
                                <DataGrid.Columns>
                                    <DataGridTextColumn Header="Wert" _
                                      Binding="{Binding einzelwert}" />
                                    <DataGridTextColumn Header="Bezeichung" _
                                    Binding="{Binding bezeichnung}" />
                                </DataGrid.Columns>
                            </DataGrid>                           
                        </DataTemplate>
                    </DataGridTemplateColumn.CellTemplate>
                </DataGridTemplateColumn>
            </DataGrid.Columns>
        </DataGrid>
 
        <DataGrid AutoGenerateColumns="False" ItemsSource="{Binding" & _
          "unterdaten}" Name="MyDG2" 
                  Margin="298,12,35,24" Height="246" _
                  HorizontalAlignment="Stretch" 
                  VerticalAlignment="Stretch" Width="267">
           <DataGrid.Columns>
                <DataGridTextColumn Header="Wert" Binding="{Binding" & _
                  "einzelwert}" />
                <DataGridTextColumn Header="Bezeichung" Binding="{Binding" & _
                "bezeichnung}" />
            </DataGrid.Columns>
        </DataGrid>
    </Grid>
</Window>