These are Céline’s creations. I gather they were not synonyms in his mind. And they do not seem to be synonyms to a French reader.

The first instance could either be a proper noun (Pirette can be a surname) or created by Céline as in insult. If we consider the context of the paragraph I would rather think it is an insult, maybe made up to rime with the preceding sornettes (a real word, 3 syllables as well). In the whole paragraph even words that are not usually insults (fripe, défroque, etc.) are used as insults. And the whole is made up for a comic effect. If it is used as an insult grammatically it is an interjection, therefore it is not a noun.

The word pyrette in the second instance is grammatically a noun (fem.). There’s no such word in French. Considering the context (poudre/au fond de and in reference to the paragraph sortilèges, philtre(ries), etc.) it could call to mind the word pipette.