To support touch operations, we can't disable HorizontalScrollMode and VerticalScrollMode. The following seems to work. Can you give it a try?

<Grid ColumnDefinitions="*,Auto">
    <Grid
        Grid.Column="0"
        ColumnDefinitions="*,*"
        PointerWheelChanged="Grid_PointerWheelChanged"
        SizeChanged="Grid_SizeChanged">
        <Grid.Resources>
            <Style BasedOn="{StaticResource DefaultScrollViewerStyle}" TargetType="ScrollViewer">
                <Setter Property="VerticalAlignment" Value="Top" />
                <Setter Property="HorizontalScrollBarVisibility" Value="Hidden" />
                <Setter Property="VerticalScrollBarVisibility" Value="Hidden" />
                <!--
                <Setter Property="HorizontalScrollMode" Value="Disabled" />
                <Setter Property="VerticalScrollMode" Value="Disabled" />
                -->
            </Style>
        </Grid.Resources>
        <ScrollViewer
            x:Name="LeftScrollViewer"
            Grid.Column="0"
            ViewChanged="LeftScrollViewer_ViewChanged">
            <TextBlock FontSize="2048" Text="Left" />
        </ScrollViewer>
        <ScrollViewer
            x:Name="RightScrollViewer"
            Grid.Column="1"
            ViewChanged="RightScrollViewer_ViewChanged">
            <TextBlock FontSize="1024" Text="Right" />
        </ScrollViewer>
    </Grid>
    <ScrollBar
        x:Name="VerticalScrollBar"
        Grid.Column="1"
        IndicatorMode="MouseIndicator"
        ValueChanged="VerticalScrollBar_ValueChanged" />
</Grid>

private void Grid_SizeChanged(object sender, SizeChangedEventArgs e)
{
    VerticalScrollBar.ViewportSize = e.NewSize.Height;
    VerticalScrollBar.Maximum = Math.Max(LeftScrollViewer.ScrollableHeight, RightScrollViewer.ScrollableHeight);
}

private void Grid_PointerWheelChanged(object sender, PointerRoutedEventArgs e)
{
    VerticalScrollBar.Value = Math.Max(0, Math.Min(this.VerticalScrollBar.Maximum, this.VerticalScrollBar.Value - e.GetCurrentPoint(this).Properties.MouseWheelDelta));
}

private void VerticalScrollBar_ValueChanged(object sender, RangeBaseValueChangedEventArgs e)
{
    if (e.NewValue < 0 || e.NewValue > this.VerticalScrollBar.Maximum)
        return;

    this.LeftScrollViewer.ScrollToVerticalOffset(e.NewValue);
    this.RightScrollViewer.ScrollToVerticalOffset(e.NewValue);
}

private void LeftScrollViewer_ViewChanged(object sender, ScrollViewerViewChangedEventArgs e)
{
    if (sender is not ScrollViewer scrollViewer)
        return;

    VerticalScrollBar.Value = scrollViewer.VerticalOffset;
}

private void RightScrollViewer_ViewChanged(object sender, ScrollViewerViewChangedEventArgs e)
{
    if (sender is not ScrollViewer scrollViewer)
        return;

    VerticalScrollBar.Value = scrollViewer.VerticalOffset;
}