In this study, a duplex model is developed as a constitutive model for the homogenised elastic–viscoplastic behaviour of plate–fin structures. This model consists of plate and fin layers, which are individually ruled by different macroscopic constitutive models. An anisotropic, compressible power law equation that was derived in a recent study by the present authors is used to describe the homogenised viscoplastic behaviour of the fin layer. On the other hand, an isotropic, incompressible power law equation is used as the macroscopic viscoplastic constitutive equation of the plate layer. The duplex model developed is applied to an ultrafine plate–fin structure made of Hastelloy X. It is thus shown that the duplex model is more successful in simulating the homogenised viscoplastic behaviour under multiaxial loading than the corresponding simplex model in which plates and fins are non‐separately homogenised.