When importing the generated dbf and shp from the README.md zip example into https://mapshaper.org/ it seems like the shapefile merges the two polylines into a singular polyline feature. I'm not educated on the internal workings of shapefiles (even after headscratchingly reading the 1998 published whitepaper), but that's my best understanding of what's happening.

I'm trying to resolve this as it prevents me from importing generated shapefiles into Autodesk Civil 3D resulting in the following error:

I do think I have a solution, will do a PR soon.