  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 (tensor{W}). 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 curve. 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
\tensor{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.
