Specifying a second ID variable provides a hierarchy of categories, but the bubbles are not split by the second category until they are selected and split interactively. In the launch window, if you specify a second ID variable, Split and Combine buttons appear in the report window.
For example, you might specify a country as the first ID variable, resulting in a separate aggregated bubble for each country. A second ID variable, perhaps designating regions within each country, would further split each country when the interactive Split button under the graph is pressed.