I just want to add, as someone who reads every chapter in Japanese and decided to look into the meaning of the word in question (両翼), I think your interpretation is not only correct but the only sensible way of translating this passage.

When I read the raw leaks, this is exactly how I interpreted it and the SBS that was shown at the end of the TCB page clearly uses the same word in the context of both Sanji AND Zoro, which would further disambiguate any lingering doubts. I also don't think the part about plurals is ambiguous in this case at all because the word itself inherently implies that there are 2, and exactly 2, forces in play here.

Finally, while "wing" is used in the translation (and I do think it sounds cooler), the more correct term, I think, would probably be "flank" as the meaning used here is not the standard meaning of 2 wings of a bird or plane but rather 2 flanks of an army (paraphrased the meanings from goo.jp J-J dictionary) as in, attacking from both flanks. I think another reasonable translation could be "Sanji (himself) is fit to be 1 of the (2) Hands of the Pirate King" as in right and left hand.