  We study a millicharged co-interacting dark matter scenario, where the
primary dark matter constituent is the dark photon $A'$ and the secondary
component is the fermion $\chi$. In this model, $\chi$ interacts with $A'$ via
a $U(1)'$ interaction while being millicharged with respect to normal photons.
Our investigation focuses on the oscillation of $A'$ dark matter into photons
within the background of $\chi$ particles, revealing that the $A'-\chi$
scattering rate benefits from a Bose enhancement of the $A'$ final state. As
the oscillation production rate is directly linked to the scattering rate, the
conversion of $A'$ dark matter into monochromatic photons experiences
significant amplification owing to this Bose enhancement, especially when the
scattering rate $\Gamma_{\rm sca}$ approaches the dark photon mass $m_{A'}$.
These converted monochromatic photons are detectable through radio telescopes
and can induce distortions in the Cosmic Microwave Background (CMB) spectrum.
We find that the sensitivity of radio telescopes and the constraints imposed by
CMB distortion on the kinetic mixing parameter are notably heightened compared
to scenarios without the subdominant millicharged dark matter.
