  We discuss duality in ``two-photon''-like processes in the scalar
$\varphi^3_E$ model and also in the process $\gamma^*\gamma\to\pi\pi$ in QCD.
Duality implies the equivalence between two distinct nonperturbative
mechanisms. These two mechanisms, one involving a twist-3 Generalized
Distribution Amplitude, the other employing a leading-twist Transition
Distribution Amplitude, are associated with different regimes of factorization.
In the kinematical region, where the two mechanisms overlap, duality is
observed for the scalar $\varphi^3_E$ model, while in the QCD case the
appearance of duality turns out to be sensitive to the particular
nonperturbative model applied and can, therefore, be used as a tool for
selecting the most appropriate one.
