Understanding Compound Dimensions
A Compound Dimension is created by combining all or selected Dimension Values from two existing Dimensions. A Metric or a Report may be Dimensioned using a Compound Dimension. As a result, an instance of the element for each Compound Dimension Values fetched in a Result Set.
When creating a new Compound Dimension, one of the Dimensions is designated as the Primary and the other as the Secondary Dimension.
Instructions for creating a new Compound Dimension are found here: Create a Compound Dimension with Selected Values