I also don’t think WL Batch Processor/watch folders can do this but I can’t be sure as I just don’t have the need for it.
There is a new batch processor app called Myriad (formerly Sample Manager) by Audiofile Engineering. It’s still in public beta (January release they say) but I believe it has a function that might do what you’re looking for.
There is an action called “mix” that could do what you’re looking for. I’d sign up for the beta and see if that’s the case.
Maybe PG could consider your request for the future if it’s not already possible. I’m not sure how widespread the use of this would be.