How can I fill the area(s) between two Series of Splines or Lines

To do that we code one of the Paint events:

Here the ValueToPixelPosition functions are valid and provide us with the necessary conversion between the DataPoint values and Chart pixels..:

private void chart1_Paint(object sender, PaintEventArgs e)
{
    // we assume two series variables are set..:
    if (sps1 == null || sps2 == null) return;

    // short references:
    Axis ax = chart1.ChartAreas[0].AxisX;
    Axis ay = chart1.ChartAreas[0].AxisY;

    // now we convert all values to pixels
    List<PointF> points1 =  sps1.Points.Select(x=>
        new PointF((float)ax.ValueToPixelPosition(x.XValue), 
                   (float)ay.ValueToPixelPosition(x.YValues[0]))).ToList();

    List<PointF> points2 =  sps2.Points.Select(x=>
        new PointF((float)ax.ValueToPixelPosition(x.XValue), 
                   (float)ay.ValueToPixelPosition(x.YValues[0]))).ToList();

    // one list forward, the other backward:
    points2.Reverse();

    GraphicsPath gp = new GraphicsPath();
    gp.FillMode = FillMode.Winding;  // the right fillmode

    // it will work fine with either Splines or Lines:
    if (sps1.ChartType == SeriesChartType.Spline )   gp.AddCurve(points1.ToArray());
    else gp.AddLines(points1.ToArray());
    if (sps2.ChartType == SeriesChartType.Spline) gp.AddCurve(points2.ToArray());
    else gp.AddLines(points2.ToArray()); 

    // pick your own color, maybe a mix of the Series colors..
    using (SolidBrush brush = new SolidBrush(Color.FromArgb(66, Color.DarkCyan)))
        e.Graphics.FillPath(brush, gp);
    gp.Dispose();
}

Note that this is not tested for zooming..