Simple. It don’t.
You can see others wanting this kind of effect in this thread:
Feature request: “Watercolor edge” (dark edge) to brush stroke
The concensus was that a good and awesome addition to Krita, one that would give this effect. While allowing much more interesting brushes was a ‘minor brush’. Basically drawing a brush inside another brush. Here a topic with the feature request:
New concept: minor brush
If you don’t want to read the topics above here is the saddest part. A ‘real’ dual brush was implemented in krita, but scrapped for a clone of Photoshop’s mask brushes.