Well, the real tricky part is that my brush keeps the width of edge constant when the size increases(I mean increase size by pressure while drawing, the edge gets wider if you press “]”). If you just put a smaller tip on the top with subtract blending mode and same pressure line, it would look like the upper line in the picture below.
The trick is to shift the pressure curve right a little bit, make sure it’s parallel to the curve of the base tip:
I just noticed that the bush preset I uploaded doesn’t strictly follow the rule, and the edge width just looks approximately constant. I made it a long time ago, maybe I have changed something and forgot about it. But now you get the point and can modify it by yourself.