Horizontal & Vertical GridSplitter Example - with two list boxes above and below the horizontal split.

Horizontal & Vertical GridSplitter Example

<Page xmlns="http://schemas.microsoft.com/winfx/2006/xaml/presentation"
 xmlns:sys="clr-namespace:System;assembly=mscorlib" 
 xmlns:x="http://schemas.microsoft.com/winfx/2006/xaml" >

<Grid >  
  <Grid.RowDefinitions>
    <RowDefinition Height="*" />
     <RowDefinition Height="Auto" />
    <RowDefinition Height="*" />
  </Grid.RowDefinitions>
  <Grid.ColumnDefinitions>
    <ColumnDefinition Width="*" />
    <ColumnDefinition Width="Auto" />
    <ColumnDefinition Width="*" />
  </Grid.ColumnDefinitions>

  <ListBox Grid.Row="0" Background="White">
    <ListBoxItem>Item 1</ListBoxItem>
    <ListBoxItem>Item 2</ListBoxItem>
    <ListBoxItem>Item 3</ListBoxItem>
    <ListBoxItem>Item 4</ListBoxItem>
    <ListBoxItem>Item 5</ListBoxItem>
  </ListBox>

  <GridSplitter Grid.Row="1" Height="3" Background="Black"
                VerticalAlignment="Center" HorizontalAlignment="Stretch" />

  <ListBox Grid.Row="2" Background="Gray" >
    <ListBoxItem>Item 1</ListBoxItem>
    <ListBoxItem>Item 2</ListBoxItem>
    <ListBoxItem>Item 3</ListBoxItem>
    <ListBoxItem>Item 4</ListBoxItem>
    <ListBoxItem>Item 5</ListBoxItem>
    <ListBoxItem>Item 6</ListBoxItem>
    <ListBoxItem>Item 7</ListBoxItem>
    <ListBoxItem>Item 8</ListBoxItem>
    <ListBoxItem>Item 9</ListBoxItem>
    <ListBoxItem>Item 10</ListBoxItem>
  </ListBox>
  
    <GridSplitter Grid.Row="0" Grid.RowSpan="3" Grid.Column="1"  Width="3" Background="Black"
                  VerticalAlignment="Stretch" HorizontalAlignment="Center" />
                  
    <Rectangle Fill="LightBlue" Grid.Column="2" Grid.Row="0" Grid.RowSpan="3" />       
</Grid>

</Page>