public class WrapStackPanel : Panel
    {
        protected override Size MeasureOverride(Size availableSize)
        {
            var width = availableSize.Width >= 501 ? availableSize.Width / 2d : availableSize.Width;
            var height1 = 0d;
            var height2 = 0d;

            for (int i = 0; i < Children.Count; i++)
            {
                var child = Children[i];
                child.Measure(new Size(width, double.PositiveInfinity));

                if (i % 2 == 1 && availableSize.Width >= 501)
                {
                    height2 += child.DesiredSize.Height;
                }
                else
                {
                    height1 += child.DesiredSize.Height;
                }
            }

            return new Size(availableSize.Width, Math.Max(height1, height2));
        }

        protected override Size ArrangeOverride(Size finalSize)
        {
            var width = finalSize.Width >= 501 ? finalSize.Width / 2d : finalSize.Width;
            var height1 = 0d;
            var height2 = 0d;

            for (int i = 0; i < Children.Count; i++)
            {
                var child = Children[i];
                var x = (i % 2 == 1 && finalSize.Width >= 501) ? width : 0;
                var y = (i % 2 == 1 && finalSize.Width >= 501) ? height2 : height1;

                child.Arrange(new Rect(x, y, width, child.DesiredSize.Height));

                if (i % 2 == 1 && finalSize.Width >= 501)
                {
                    height2 += child.DesiredSize.Height;
                }
                else
                {
                    height1 += child.DesiredSize.Height;
                }
            }

            return new Size(finalSize.Width, Math.Max(height1, height2));
        }
    }