Running two apps side by side with preferences?

Essentially, this can only be done if the apps have different signatures [I think they're now UTIs, used to be Creator Codes].

Persuading an app to run twice can be as simple as duplicating it in the Finder, but persuading it to use a different set of prefs means being able to 'fool' the OS into thinking it is, indeed, a separate app, with its own signature & prefs naming convention.
This can only typically be done at compile, which of course requires you have the source code - these days hindered further by having to have valid developer certificates.

In the days of BBEdit having a 'junior' app, TextWrangler, this would have been easy.