I’m reading the Symposium with Dover’s notes. I was rather confused when 187cd earlier and started writing out this post to ask what was going on. However, writing out the below translation seems to have improved my understanding. So instead I’ll ask for people to take a look and tell me if I’m getting anything wrong.
And in the construction itself of harmony and rhythm there is no difficulty of distinguishing the love-elements, nor is the two-fold eros anywhere there, but whenever it is necessary to make full use of rhythm and harmony for an audience or for one to compose (what they call songwriting) or to correctly use already created melodies and meters (what they call culture or education), now there is difficulty and it needs a good technician.
ἢ ποιοῦντα … ἢ χρώμενον ὀρθῶς τοῖς πεποιημένοις μέλεσί τε καὶ μέτροις,
The first η is “either”, not “or”. And these are participles, in reference to one who has to use rhythm and music (δέῃ πρὸς τοὺς ἀνθρώπους καταχρῆσθαι ῥυθμῷ τε καὶ ἁρμονίᾳ)—either as composer or performer.
And in the note to 176d4 he says “the accusative of the participle is used, not a dative agreeing with ἄλλῳ, because the analysis is not ‘I couldn’t advise another, when he has a hangover, to drink deep’ but 'I couldn’t advise another that one should drink deep when one has a hangover.”
The accusatives are the regular accusative you get with impersonal δεῖ. “When the composer has to …” wd be ἐπειδὰν δέῃ τὸν ποιοῦντα …, but here the participles are predicative not attributive. As I’ve said I don’t know how many times before, “it is necessary” is never a good translation for δεῖ. Better “when one has to use … either when composing or when …”, or e.g. “when rhythm and harmony are to be used either in composition or in performance.”