  We propose a novel mechanism with two component dark matter models. The
subdominant dark matter can thermalize the dominant one in galaxies, and leads
to core density profiles. Using ultralight dark photons and
$\mathrm{GeV}$-$\mathrm{TeV}$ Dirac fermions as an example, we couple the two
dark matter candidates with a $U(1)$ interaction. This mechanism differs from
self-interacting dark matter, due to three effects: (1) higher occupation
numbers, (2) forward-backward scattering cancellation, and (3) the multiple
scatterings required for the heavy dark matter. Furthermore, the Bullet Cluster
bound is evaded due to the reduced Bose enhancement factor. Unlike the fuzzy
dark matter solution to the small structure problems which have tension with
Lyman-$\alpha$, the ultralight dark photons with mass $\gtrsim 10^{-21}$ eV can
have a core profile through interactions with $\psi$ and are not constrained by
other astrophysical observations.
