  A two-variable stochastic model for diffusion-limited nucleation is developed
using a formalism derived from fluctuating hydrodynamics. The model is a direct
generalization of the standard Classical Nucleation Theory. The nucleation rate
and pathway are calculated in the weak-noise approximation and are shown to be
in good agreement with direct numerical simulations for the
weak-solution/strong-solution transition in globular proteins. We find that
Classical Nucleation Theory underestimates the time needed for the formation of
a critical cluster by two orders of magnitude and that this discrepancy is due
to the more complex dynamics of the two variable model and not, as often is
assumed, a result of errors in the estimation of the free energy barrier.
