We present an analytical framework capable of describing spin-waves dynamic in magnetic hetero-structures composed of a pair of exchange-decoupled magnetic layers separated by a nonmagnetic spacer, focusing in particular on garnet-based multilayers. The model captures the formation of collective spin-wave modes, namely symmetric and antisymmetric, arising from dipolar coupling and provides direct access to the dispersion relation of the system and consequent interference phenomena. This formalism establishes a versatile theoretical tool for the predictive design of dipolarly coupled magnonic devices, providing access to their eigenfrequencies and mode shapes.