Discrete time state-space models for multiple source frequency modulated communications environments are developed and coupled digital phase-locked loop (CDPLL) estimator structures are derived based on the extended Kalman filter (EKF). Unlike related efforts found in the literature, the receiver presented includes both passband and baseband coupling between the DPLLs. Separability of two sources is investigated by examination of state observability and by simulations of the derived estimators. The relationship between the EKF and the CDPLL is presented and their linear and nonlinear behavior discussed. As the CDPLLs exhibit chaotic behavior during acquisition and cannot be linearized during tracking, acquisition and tracking characteristics are extracted from simulations. One simulation result is the effect of state observability on the coupling between the DPLLs; when the states are "strongly" observable, the states are not as tightly coupled. It is also shown that the loop bandwidths decrease during periods of "weak" observability.