Bivariate maps are the geographical equivalent of Boolean math. Through the comparison of two related variables we can visualized the regions where both conditions are true (AND) as well as where one is true and the other is false (OR). To make a bivariate choropleth map, you should overlay the two variables. This can be done by selecting the number of classes for each choropleth (usually 2 or 3) and applying a quantile classification. Then you assign each variable with a scaled value (1,2,3... A,B,C...) and combine to produce a new field withe 4 or 9 types. When you symbolize based on these types with a predetermined color scheme, the end result should give the appearance of overlaying the two maps.
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