  In this work, we establish a regularity theory for the optimal transport
problem when the target is composed of two disjoint convex domains, denoted
$\Omega^*_i$ for $i=1, 2$. This is a fundamental model in which singularities
arise. Even though the singular set does not exhibit any form of convexity a
priori, we are able to prove its higher order regularity by developing novel
methods, which also have many other interesting applications (see Remark 1.1).
Notably, our results are achieved without requiring any convexity of the source
domain $\Omega$. This aligns with Caffarelli's celebrated regularity theory
\cite{C92}.
