  Integrated photonic circuits offer a promising route for studying coherent
cooperative effects of a controlled collection of quantum emitters. However,
spectral inhomogeneities, decoherence and material incompatibilities in the
solid state make this a nontrivial task. Here, we demonstrate efficient
coupling of a pair of organic molecules embedded in a plastic film to a TiO$_2$
microdisc resonator on a glass chip. Moreover, we tune the resonance
frequencies of the molecules with respect to that of the microresonator by
employing nanofabricated electrodes. For two molecules separated by a distance
of about 8$\,\mu$m and an optical phase difference of about $\pi/2$, we report
on a large collective extinction of the incident light in the forward direction
and the destructive interference of its scattering in the backward direction.
Our work sets the ground for the coherent coupling of several molecules via a
common mode and the realization of polymer-based hybrid quantum photonic
circuits.
