  The (tree) amplituhedron $\mathcal A_{n,k,m}(Z)$ is a certain subset of the
Grassmannian introduced by Arkani-Hamed and Trnka in 2013 in order to study
scattering amplitudes in $N=4$ supersymmetric Yang-Mills theory. Confirming a
conjecture of the first author, we show that when $m$ is even, a collection of
affine permutations yields a triangulation of $\mathcal A_{n,k,m}(Z)$ for any
$Z\in \operatorname{Gr}_{>0}(k+m,n)$ if and only if the collection of their
inverses yields a triangulation of $\mathcal A_{n,n-m-k,m}(Z)$ for any
$Z\in\operatorname{Gr}_{>0}(n-k,n)$. We prove this duality using the twist map
of Marsh and Scott. We also show that this map preserves the canonical
differential forms associated with the corresponding positroid cells, and hence
obtain a parity duality for amplituhedron differential forms.
