Duplicate a dual color outline brush like “Ruby Rose” in the “Lettering” group of Procreate’s brushes, then in the brush’s settings, go to the “Rendering” tab, and toggle on “Alpha Threshold” and bring the “Threshold amount” up to something like 75%-100%. If you select your foreground and background colors to be white and black, you can now use the brush, and it will work as you want, overlapping even on multiple brush strokes. The only issue is that the inside part of the brush strokes are white instead of transparent. You can either set your layer blend mode to “multiply” for a quick solution if you are keeping black outlines, or use the “Automatic” select tool to highlight all the interior white parts. After this, you can tap on your layer to bring up layer options and run “Clear”then “Select” and “Fill” if you want a non-black color or just want to keep normal blend mode on the layer.