The combination of different materials is prevalent in nature and technology. Particularly, smart (active) materials must be combined with other materials to harness their full potential. Three combination cases are explored: Case I (constrained), Case II (combined) and Case III (free) behavior. Based on analytical theories and Finite Element simulations with the Stimulus-Expansion-Model, design approaches for active-passive composites are proposed. These include, choice of Case for an application, bottom-up design of layers with matching stiffness and replacement of materials. The current work substantiates that the stiffnesses are crucial for combining materials, e.g., in smart structures, additive manufacturing and bioelectronic interfaces.