In Spatial Modulation (SM), the information bits are split into two streams: one is mapped to a conventional complex signal modulation, and the other is used to select the active transmit antenna. Transmission SM schemes can be implemented with a single RF chain, allowing for a low-complexity transmitter with relatively large spectral efficient. In this paper, we are concerned with coded SM schemes. In the literature, the usual approach considers a single encoder that jointly encodes both bit streams. In this work, we propose a coded SM system in which each of the information streams is encoded by a possibly different low-density parity-check (LDPC) code. The two LDPC codes are designed after selecting the code rates according to the mutual information associated with the two corresponding subchannels, for a fixed, reference signal-to-noise ratio (SNR). A two-layer, iterative message-passing decoder is developed, where the two component decoders exchange soft information. Through Monte Carlo simulations, we show that the bit error rate of the proposed scheme is low when the SNR is greater or equal to the reference SNR, demonstrating the strength of the proposed two-layer approach.