The problem of optimum distribution of the available spatial degrees of freedom among two sparse antenna subarray beamfomers in shared aperture receiver is investigated. The two subarrays, forming a full array, co-exist on the same platform and could perform separate RF sensing and communications tasks. The sparsity and cardinality of the subarray configurations are joint optimization variables which considerably affect the output signal-to-interference plus noise ratios (SINR) of the two beamformer outputs. A minimum output SINR figure value is imposed to guarantee minimum performance. We solve this problem by utilizing Taylor series approximation to reformulate the initial non-convex problem to a convex one. Simulation results validate the effectiveness of the proposed method.