How to make a sequencer that covers both stderr and stdout?

I have a program that:

* Loops over a set of files doing some operation
* Writes results as CSV rows to stdout
* Writes progress and error information to stderr

Example usage of the program would be: `my-tool --some-flags -- /path/to/files/* > output/findings.csv`

To keep findings comparable, I'd like to avoid interleaving CSV rows from different files, and I'd like the stderr output to avoid being interleaved to make troubleshooting straightforward. Is there a way to create a `Sequencer` that handles both?