What is the rule for using the "passive se" (e.g. "¿Cómo se dice?") with a reflexive verb that involves another se pronoun? For example, how would you translate "One takes a shower (ducharse) frequently in hot weather."? Do the two se pronouns simply combine into one?
The two pronouns just combine in one:
"One takes a shower frequently in hot weather" --> Se ducha frecuentemente cuando hace calor.
She takes a shower frequently in hot weather --> (Ella) se ducha frecuentemente cuando hace calor.
I don't understand it either. You use "se" mostly when the subject performs actions on itself or if you want to make an impersonal statement like "It's said" --> "Se dice".
As far as I know there is no "passive se".
As others have introduced, there are mainly two uses for "se".
However, in the case of reflexive verbs, the third person changes and "se" is used in the third person (both singular and plural) instead of the corresponding "le" or "les".
If you speak German, also a Germanic language like English, this case applies exactly in the same way:
I guess your doubt is the following: can a reflexive verb (which uses the "se" particle) be combined with the impersonal "se" (what you call "the passive se") in the same sentence?
Hope this helps.
I'm posting a new answer since I misunderstood and needed some more room than a comment.
In English you use an auxiliary verb "do" in "How does one shave?", but in Spanish you don't. There is only one verb in the sentence "¿Cómo se afeita uno?". We could get in an deep debate about where that "se" comes from, but I think it's safe and easy to just assume it's a fixed impersonal particle. Thinking it is a verb does not help and confuses more.
The first "se" is therefore just the "impersonal se".
Think about how you ask impersonal "how to" questions with "se" in Spanish:
That's just a fixed formula.
A small break here that hope clarifies the thing about "se" in reflexive verbs, which is not so special after all.
In a general sentence we have a subject + verb (action) + direct object (what) + indirect object (receiver).
As you know, you can substitute both direct and indirect object for pronouns.
Which in Spanish can be tricky because not only is the order of the sentence altered but there is also an exception. This is the case of the indirect object "le" (to him / to her) that changes to "se" when both a direct and indirect object pronouns are present in the same sentence.
If you only use a pronoun for the indirect object, you say:
But if you use both pronouns, instead of the logical:
Which is incorrect, "le" is changed to avoid the le-la "cacophony" (bad sound):
This is where the infamous third person "se" for the indirect object comes from.
Reflexive verbs are not so special after all. It just happens that the receiver of the action (indirect object) is the same as the performer (subject), so logically speaking they are simply a normal verb with an indirect object pronoun sticked to them to mark that they can be applied to oneself.
"Ducharse" is just a special case of "duchar" (you can "duchar a alguien" / shower somebody). Ducharse just means that this verb can be applied to oneself, and the one who's being showered is the same as "the showerer" :):
But that sounds weird so we just use reflexive pronouns (which as you see have a lot to do with indirect object personal pronouns), that in the third person is "se" to clarify that te showering is to oneself, not to a third person.
And now, back to the original answer.
Remember the general impersonal form of asking "how to"?
Why should reflexive verbs be special? We just say "uno" (meaning "un hombre", "uno a sí mismo") just like in English you say "oneself".
If it's referred specifically to a girl, then "una" is generally used:
Pfew. Hope it's clear now.
I just wanted to make clear where the "reflexive se" comes from and what the relation is.