This sounds really cool!
You can order concentric/dual pots from most guitar parts suppliers. Here’s one:
https://www.stewmac.com/electronics/components-and-parts/potentiometers/fender-dual-500k-250k-split-shaft-potentiometer.html?utm_source=google&utm_medium=shopping&utm_campaign=2021-05-gp&gclid=Cj0KCQjwkZiFBhD9ARIsAGxFX8AA5pPyl3Zb0vyHOkPyB9CDkXrB_nKZRGXQPPtD6TsLpzprvO_L3qsaAjjLEALw_wcB
I’m not sure if that’s quite what you’re looking for.
Regarding open source hardware/design I think most people just post their schematics and documents on GitHub with a permissive license so they can be revisioned and contributed to by the open source community. Here are some examples: open-source-hardware · GitHub Topics · GitHub