We consider a melt of two chemically identical homopolymers of different lengths. Both species obey Rouse dynamics, and always form a homogeneous mixture at equilibrium. Under shear flow in a wide-gap Couette, such a system exhibits strongly inhomogeneous steady states, that resemble macro-phase separated blends. The corresponding velocity profile is nonmonotonic and exhibits distinct shear bands. These findings may have implications for the processing of polydisperse polymer melts.