In multi-component soft matter, interface properties often play a key role in determining the properties of the overall system. The identification of the internal dynamic structures in non-equilibrium situations requires the interface rheology to be characterized. We have developed a method to quantify the rheological contribution of soft interfaces and evaluate the dynamic modulus of the interface. This method reveals that the dynamic shear responses of interfaces in bilayer systems comprising polypropylene and three different polyethylenes can be classified as having hardening and softening effects on the overall system: a interface between linear long polymers becomes more elastic than the component polymers, while large polydispersity or long-chain-branching of one component make the interface more viscous. We find that the chain lengths and architectures of the component polymers, rather than equilibrium immiscibility, play an essential role in determining the interface rheological properties.