<Window x:Class="WpfApplication2.MainWindow"
        xmlns="http://schemas.microsoft.com/winfx/2006/xaml/presentation"
        xmlns:x="http://schemas.microsoft.com/winfx/2006/xaml"
        Title="MainWindow" Height="350" Width="525">
    <Grid>
        <Grid.RowDefinitions>
            <RowDefinition Height="162*" />
            <RowDefinition Height="149*" />
        </Grid.RowDefinitions>
        <Grid.ColumnDefinitions>
            <ColumnDefinition Width="239*" />
            <ColumnDefinition Width="264*" />
        </Grid.ColumnDefinitions>

        <Button Grid.Column="0" Grid.Row="0" Background="Red" />
        <Button Grid.Column="1" Grid.Row="0" Background="Green" />
        <Button Grid.Column="0" Grid.Row="1" Background="Blue" />
        <Button Grid.Column="1" Grid.Row="1" Background="Yellow" />
        <GridSplitter x:Name="_splitter" Grid.Row="1" Grid.ColumnSpan="2"  Name="gridSplitter1" Height="10"
                      HorizontalAlignment="Stretch"
                      ResizeDirection="Rows" VerticalAlignment="Top" />
    </Grid>
</Window>

        <GridSplitter x:Name="_splitter" Grid.Row="1" Grid.ColumnSpan="2"  Name="gridSplitter1" Height="10"
                      HorizontalAlignment="Stretch"
                      ResizeDirection="Rows" VerticalAlignment="Top" />