Mixing of two-level unstable systems

Unstable particles can be consistently described in the framework of quantum field theory Starting from the full S matrix amplitudes of B+ --> (2 pi, 3 pi)l(+)nu decays as examples of processes where the rho-omega resonances dominate, we propose a prescription for the mixing of two "quasi-physical": unstable states that differs from the one obtained from the diagonalization of the M - i Gamma/2 non-Hermitian Hamiltonian. We discuss some important consequences for CP violation in the K-L - K-S system.