We investigate the charge and spin response functions of a bilayer quasi-two-dimensional system, spin polarized by a constant magnetic field B ជ . Terms beyond the random-phase approximation, the exchange and correlation interactions, are introduced by using generalized spin-dependent local field factors, G␴ x,c(qជ,␻). The self-consistent magnetic interaction among the electron spins determines the coupling of the charge and the longitudinal spin-density excitations, leading to coupled in-phase and out-of-phase electric and magnetic modes. We find that the lowest frequency belongs to an acoustic mode, that represents the out-of-phase oscillation of the longitudinal magnetizations in the two layers. This collective excitation is shown to become important in the case of materials with large gyromagnetic factors, such as dilute magnetic semiconductors.