arcgis desktop - Using multiple attributes feature for symbology in ArcMap?

I am trying to make a map using US census data showing vacant housing to total housing on the same map.

I came across the "multiple attributes" feature under the symbology tab and wondered if this is the way to go and, if so, how to use it?

I have only been able to get one type of symbol, the gradient feature to appear.

I'm using ArcGIS Desktop 10.