  We perform a theoretical analysis of the semileptonic decays $\eta^{(\prime)}
\to \pi^0 \ell^+ \ell^-$ and $\eta' \to \eta \ell^+ \ell^-$, where $\ell = e,
\mu$, via a charge-conjugation-conserving two-photon mechanism. The underlying
form factors are modeled using vector-meson dominance, phenomenological input,
and $\mathrm{U}(3)$ flavor symmetry. We consider both a monopole and a dipole
model, the latter tailored such that the expected high-energy behavior is
ensured. Furthermore, we benchmark the effect of $S$-wave rescattering
contributions to the decays. We infer significant effects of the form factors
neglected in the literature so far, still finding branching ratios of the
various decays well below the current experimental upper limits.
