Arrays of closely-spaced antennas with mutual coupling have been considered recently with analogies to the hearing mechanism in small insects that exhibit excellent direction finding capabilities. We develop a model for a two-element array system that includes three distinct noise sources and a 4-port electrical network that couples the antennas to the measurement loads. The optimum coupling network that minimizes the Cramer-Rao bound (CRB) for angle of arrival (AOA) estimation is derived and a design method is presented to synthesize the network. A compact expression for the CRB provides insights about the fundamental value of mutual coupling and the importance of an optimum coupling network, and we show simulation results for an AOA estimator that achieves the CRB.