Here I explain how to combine one report (crosstab) with multiply data sources by a dropdown list.
After you have create a new report you have to create a crosstab, a dropdown list and several datasources:
You click on the dropdown list and create on Items the values which you want to select. Each value we will be mapped to one datasource.
After that you select Events/on select you have to map the Dropdown list with the data sources.