[Leaving this as a note to myself to circle back.]
I’m just going to say it: some “rules” are probably two or three rules, because the idea that rules are defined by natural classes (and thus free of disjunctions) is more entrenched than our intuitions about whether or not a process in some language is really one rule or not, and we should be Gallilean about this. Here are some phonological “rules” that are probably two or three rules different rules.
- Indo-Iranian, Balto-Slavic families, and Albanian “ruki” (environment: preceding {w, j, k, r}): it is not clear to me if any of the languages (the) actually need this as a synchronic rule at all.
- Breton voiced stop lenition (change: /b/ to [v], /d/ to [z], /g/ to [x]): the devoicing of /g/ must be a separate rule. Hat tip: Richard Sproat. I believe there’s a parallel set of processes in German.
- Lamba patalatalization (change: /k/ to [tʃ], /s/ to [ʃ]): two rules, possibly with a Duke-of-York thing. Hat tip: Charles Reiss.
- Mid-Atlantic (e.g., Philadelphia) English ae-tensing (environment: following tautosyllabic, same-stem {m, n, f, θ, s, ʃ]): let’s assume this is allophony; then the anterior nasal and voiceless fricative cases should be separate rules. It is possible the incipient restructuring of this as having a simple [+nasal] context provides evidence for the multi-rule analysis.
- Latin glide formation (environment: complex). Front and back glides are formed from high short monophthongs in different but partially overlapping contexts.