We present an architecture tailored for the multiplexing of multiple optical chaotic carriers generated by semiconductor lasers with external optical cavities. Our setup can discriminate multiple chaotic signals with high spectral overlap. The various emitters are mutually globally coupled thanks to a shared optical feedback, which creates a multiplexed optical field. This field is then coherently and unidirectionally injected in the decoupled receivers, and allows each of them to synchronize on their respective emitter. Using this setup, it would be possible to transmit several messages and make a better use of the wide chaotic spectrum. In this paper, we demonstrate theoretically and numerically the possibility to synchronize two optical chaotic fields as a premise for the transmission of two messages. We also study the robustness of synchronization to parameter mismatch and noise, which are important issues in real field experiments.