I've just found this paper: K. O'Nolan, Doublets in the Odyssey, which I've encountered in Cambridge G&L commentary by accident, but it reminded me that discussion about μῦθος καὶ ἔπος of the other day.
This is what I understood: according to the author, formulaic doublets (i.e: words that are essentially synonyms but that are used together in the same formula) are essentially the same kind of formulae that are noun-epithets formulae. Noun-epithets are tautological, when Homer says σπέσσι γλαφυροῖσι he is basically saying hollow hollows, and so are doublets according to the author: μύθος καὶ ἔπος would be, accepting that, a similar formula by which a single idea is expressed by two images, the same as any other noun-epithet formula. I understand that there is a lot writing about noun-epithets formulae, so I suppose that by this reductionism one could think about formulaic doublets as the same poetic device that noun-epithets are.
Here is the paper if someone is interested of how far might I distort the original idea of the author:
http://www.jstor.org/discover/10.2307/6 ... 4254862547