In this paper, we present a new type of Spatial Multiplexing (SMX) schemes based on the multiple signal constellation concept, which was recently introduced in the context of Spatial Modulation (SM) by the present authors. The proposed technique, which we refer to as Enhanced SMX or E-SMX, conveys information not only by the transmitted symbols, but also by the antenna and constellation combinations used. In addition to the primary constellation, these schemes make use of one or more specifically-designed secondary constellations, obtained through geometric interpolation in the signal constellation plane. We present the general concept and describe specific schemes for different numbers of transmit antennas and using 16QAM as primary modulation. Our analysis and the simulation results indicate that the proposed schemes provide a significant performance gain over conventional SMX.