Before revision:  Our identification strategy requires that the set of available proxy controls be split into two subsets, one subset acting as an instrument for the other.
After revision:  Our identification results motivate an easy-to-implement nonparametric estimation method. Under our identifying assumptions the estimation problem is `well-posed'. We describe the estimator and derive convergence rates. When applied in panel settings our methods provide a novel approach to identification with non-separable general heterogeneity and a fixed time dimension.