Data Fusion of wireless sensors is a common technique employed in many communication systems. This work focuses on incorporating the principles of non-orthogonal-multiple-access (NOMA) to optimize error performance directly in the choice of constellation design. More specifically, the problem of two sensor data fusion of a binary uniform source sent over a Gaussian multiple access channel via symmetric binary constellations is investigated. A so-called planar upper bound on the error probability is analytically derived. A constellation design is then obtained by establishing in closed form its rotation parameter that minimizes the upper bound. Simulation results show that the resulting constellations achieve a near identical performance as experimentally determined optimal constellations.