To combine queries from different data sources, the data source from the Primary Query must be compatible with the data sources from any other queries. Look to the chart below. Note that the arrows point in the direction of compatible data sources. For example, if you use the PO data source for your Primary Query, you can add a PO Manifest Item fact as a compatible query.
Sample: Primary Query - Manifest
If you were to use the Manifest data source as your Primary Query, you could use any of the following data sources as secondary queries.