We develop a simple phenomenological theory to describe linear viscoelasticity in bidisperse linear polymer melts. We describe the single-chain relaxation spectrum using a local description of relaxation times along the chain, which includes contour-length fluctuations as well as reptative motion. The complex modulus is calculated by summing the contributions from all the segments along the chain, and weighting the contributions by the entangled volume fraction remaining at each time. We find that the resulting predictions for the modulus fit data of binary blends of polybutadiene better than those of the widely used double reptation model.