Code

private void fc_ArrowCreated(object sender, MindFusion.FlowChartX.ArrowEventArgs e)
{
    ArrowCollection commonArrows = getCommonArrows(e.Arrow.Origin, e.Arrow.Destination);

    PointF pt1 = e.Arrow.ControlPoints[0];
    PointF pt2 = e.Arrow.ControlPoints[e.Arrow.ControlPoints.Count - 1];

    if (commonArrows.Count > 1)
    {
        for (int c = 0; c < commonArrows.Count; ++c)
        {
            Arrow arrow = commonArrows[c];
            arrow.Style = ArrowStyle.Bezier;
            arrow.SegmentCount = 1;

            PointF cp1 = new PointF(pt1.X + 1 * (pt2.X - pt1.X) / 3, pt1.Y + 1 * (pt2.Y - pt1.Y) / 3);
            PointF cp2 = new PointF(pt1.X + 2 * (pt2.X - pt1.X) / 3, pt1.Y + 2 * (pt2.Y - pt1.Y) / 3);

            float angle = 0, radius = 0;
            cartesianToPolar(pt1, pt2, ref angle, ref radius);

            int pairOffset = (c / 2 + 1) * 5;
            if (commonArrows.Count % 2 == 0)
            {
                polarToCartesian(cp1, c % 2 == 0 ? angle - 90 : angle + 90, pairOffset, ref cp1);
                polarToCartesian(cp2, c % 2 == 0 ? angle - 90 : angle + 90, pairOffset, ref cp2);

                if (arrow.ControlPoints[0] == pt1)
                {
                    arrow.ControlPoints[1] = cp1;
                    arrow.ControlPoints[2] = cp2;
                }
                else
                {
                    arrow.ControlPoints[1] = cp2;
                    arrow.ControlPoints[2] = cp1;
                }

                arrow.UpdateFromPoints();
            }
        }
    }
}

ArrowCollection getCommonArrows(Node box1, Node box2)
{
    ArrowCollection commonArrows = new ArrowCollection();

    foreach (Arrow arrow in box1.OutgoingArrows)
        if (arrow.Destination == box2)
            commonArrows.Add(arrow);

    foreach (Arrow arrow in box1.IncomingArrows)
        if (arrow.Origin == box2)
            commonArrows.Add(arrow);

    return commonArrows;
}

void polarToCartesian(PointF coordCenter, float a, float r, ref PointF cartesian)
{
    if (r == 0)
    {
        cartesian = coordCenter;
        return;
    }

    cartesian.X = (float)(coordCenter.X + Math.Cos(a * Math.PI / 180) * r);
    cartesian.Y = (float)(coordCenter.Y - Math.Sin(a * Math.PI / 180) * r);
}

void cartesianToPolar(PointF coordCenter, PointF cartesian, ref float a, ref float r)
{
    if (coordCenter == cartesian)
    {
        a = 0;
        r = 0;
        return;
    }

    float dx = cartesian.X - coordCenter.X;
    float dy = cartesian.Y - coordCenter.Y;
    r = (float)(Math.Sqrt(Math.Pow(dx, 2) + Math.Pow(dy, 2)));

    a = (float)(Math.Atan(-dy / dx) * 180 / Math.PI);
    if (dx < 0)
        a += 180;
}