Is it possible to have an igDataChart with 2 series where one completely overlays the other as shown in the image attached?
Yes, you can achieve this. To do so you should set the overlap on x axis – this property accepts values from -1 to 1. Setting value of 0 will put the columns next to each other. Setting negative value will increase the space between columns. Setting positive value will overlap the columns. In your case you should set this property to 1.
Here is a JSFiddle showing how you can achieve this.
Unfortunately, this does not achieve the desired result. The series columns do overlap, but one completely hides the other so that they look like stacked columns. One of the series columns must be narrower than the other so that you see both from bottom to top for each column.
Setting the width of the columns per series is not supported in igxDataChart. However, you can easily achieve what you need by adding second X axis to your chart. Then set different overlap to the x axes and assign each series its own x axis.
I have updates my JSFiddle showing this approach.