As you can see the Mergecontent processor has several parameters for ensuring messages with a commonality get merged. https://nifi.apache.org/docs/nifi-docs/components/org.apache.nifi/nifi-standard-nar/1.6.0/org.apache.nifi.processors.standard.MergeContent/index.html   This could be used in the way that you let each of the two processors give the messages an attribute, and only merge things with the same attribute.   You appear to be looking for the opposite. I am not aware of any such parameter, and am also not sure if this is in the spirit of what mergecontent was designed for. Perhaps it is time to step back and see if mergecontent (or even NiFi) is the best tool for this specific job. ... View more