To reproduce:

AreaSeries areaSeries = new AreaSeries();
areaSeries.DataPoints.Add(new CategoricalDataPoint(13, "Jan"));
areaSeries.DataPoints.Add(new CategoricalDataPoint(20, "Apr"));
areaSeries.DataPoints.Add(new CategoricalDataPoint(15, "Jul"));
areaSeries.DataPoints.Add(new CategoricalDataPoint(16, "Oct"));
this.radChartView1.Series.Add(areaSeries); 

areaSeries.BackColor = Color.Transparent;

areaSeries.CombineMode = ChartSeriesCombineMode.Stack;
AreaSeries areaSeries2 = new AreaSeries();
areaSeries2.DataPoints.Add(new CategoricalDataPoint(15, "Jan"));
areaSeries2.DataPoints.Add(new CategoricalDataPoint(25, "Apr"));
areaSeries2.DataPoints.Add(new CategoricalDataPoint(27, "Jul"));
areaSeries2.DataPoints.Add(new CategoricalDataPoint(18, "Oct"));
this.radChartView1.Series.Add(areaSeries2);
areaSeries2.CombineMode = ChartSeriesCombineMode.Stack;

 The area underneath the transparent one shouldn't be displayed in this case because they are stacked.  There is no area underneath the bottom layer to display. They are each STACKED on top of each other, they should not be layered in front of each other.