  We study a long-overlooked CP violation observable, termed double-mixing CP
violation, which arises from the interference between two neutral meson
oscillating paths involved in a decay chain. The double-mixing CP violation is
beneficial for the precise test of the Standard Model CKM mechanism, as it
offers the potential to extract weak phases without pollution from strong
dynamics. To provide a comprehensive understanding of the double-mixing CP
violation, we perform phenomenological analyses on the cascade decays of the
$B^0_d$ and $B^0_s$ mesons. Our results show that the double-mixing CP
violation can be very significant in certain decay channels, such as $B^0_s \to
\rho^0 K \to \rho^0(\pi^-\ell^+\nu_\ell)$. In addition, we employ the decay
process $B^0_d \to D^0 K \to (K^-\pi^+)(\pi\ell\nu)$ to demonstrate that the
involved strong and weak phases can be directly determined from the
experimental data without any theoretical inputs.
