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

[TedDriggs]

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?