We study the early stages of the shear banding instability in semidilute wormlike micelles using the non-local Johnson-Segalman model with a two-fluid coupling of the concentration (phi) to the shear rate (gamma_dot) and micellar strain (tensorW). We calculate the "spinodal" limit of stability for sweeps along the homogeneous intrinsic flow curve. For startup "quenches" into the unstable region, the instability in general occurs before the homogeneous startup flow can attain the intrinsic flow

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... urve. We predict the selected time and length scales at which inhomogeneity first emerges. In the "infinite drag" limit, fluctuations in the mechanical variables (gamma_dot and W) are independent of those in phi, and are unstable when the slope of the intrinsic flow curve is negative; but no length scale is selected. For finite drag, the mechanical instability is enhanced by coupling to phi and a length scale is selected, in qualitative agreement with recent experiments. For systems far from an underlying zero-shear demixing instability this enhancement is slight, while close to demixing the instability sets in at low shear rates and is essentially demixing triggered by flow.