GridSplitter

 1    <Grid>
 2             <Grid.ColumnDefinitions>
 3                 <ColumnDefinition Width="*" />
 4                 <ColumnDefinition Width="5" />
 5                 <ColumnDefinition Width="*" />
 6             </Grid.ColumnDefinitions>
 7             <TextBlock FontSize="55" HorizontalAlignment="Center" VerticalAlignment="Center" TextWrapping="Wrap">Left side</TextBlock>
 8             <GridSplitter Grid.Column="1" Width="5" HorizontalAlignment="Stretch" />
 9             <TextBlock Grid.Column="2" FontSize="55" HorizontalAlignment="Center" VerticalAlignment="Center" TextWrapping="Wrap">Right side</TextBlock>
10         </Grid>