What's the noun for duality? Here's the word you're looking for.
dual
Of an item that is one of a pair, the other item in the pair.
(geometry) Of a regularpolyhedron with V vertices and F faces, the regular polyhedron having F vertices and V faces.
(grammar)dual number The grammatical number of a noun marking two of something (as in singular, dual, plural), sometimes referring to two of anything (a couple of, exactly two of), or a chirality-marked pair (as in left and right, as with gloves or shoes) or in some languages as a discourse marker, "between you and me". A few languages display trial number.
(chemistry) The theory, originated by Lavoisier and developed by Berzelius, that all definite compounds are binary in their nature, and consist of two distinct constituents, themselves simple or complex, and having opposite chemical or electrical affinities.