Superposed constellation for spatial multiplexing visible light communication (VLC) systems has recently attracted significant attention as a promising method to alleviate the effect of nonlinearity on light-emitting diodes (LEDs) that has achieved significant performance improvements in VLC systems. A large minimum Euclidean distance (ED) value of the superposed signals can be achieved at the receiver side by employing optimal power ratios transmitted from different LEDs. However, the power allocation strategy using fixed shaped sub-constellations at the LEDs has the inherent limitation of low optimization dimensions. In this paper, we propose optimizing the sub-constellations at individual LEDs instead of just the power allocation coefficients to further increase the minimum ED value. Moreover, intra-ED and inter-ED terms are used to reduce the complexity of the optimization solving process. The simulation results show that the proposed constellation design scheme can improve the symbol error rate performance compared to the conventional one, and the right choice of pre-defined shape of the sub-constellations at the LEDs has an important role in the optimization process.