Bar in bar charts are exactly as described. Essentially a way of comparing measures against each other in a bar chart format.
Guess who? Yep! Andy Kriebel comes up trumps again.
decisivedata.net posted a nice, clear overview here:
The blog post from Andy Kriebel to partner his video is listed here:
The Information Lab shared this workbook on Tableau Public, with one tab ‘Bar in bar – using single axis’ demonstrating this chart type well:
The second example is yet another #MakeoverMonday submission, this time from Sean Miller, with bar in bar charts used on the second story point.