Data extracted from several sources can be merged together by creating a combined source.
A combined source can depend on a list of tags and/or a list of sources.
<combinedSource
name="aCombinedSource">
<dependencies>
<tag>aTag</tag>
<tag>anotherTag</tag>
<source>aSource</source>
<source>anotherSource</source>
<dependencies>
<activate name="mailFiler" />
</combinedSource>
A combined source cannot depend on another combined source.
The merged data aren't persisted anywhere - they just live during the Curiosity execution.