Based on a novel multiplexing system of two distinct chaotic signals, the corresponding modified Lang-Kobayashi rate equations are established. The numerical investigations into the performance of chaos synchronization are carried out. In more detail, the influences of single-parameter mismatch, multi-parameter mismatch, feedback-strength discrepancy, and frequency detuning between the two master semiconductor lasers (MLs) on synchronization performance are investigated, respectively. Moreover, the security and spectrum characteristics are addressed briefly in this work. The numerical simulations show that by adopting parameter mismatch, i.e., choosing appropriate system parameters of the two MLs, the correlation between the two MLs becomes extremely low, while the matched master-slave laser pairs can achieve high-quality chaos synchronization, indicating that the condition of optical chaos multiplexing is satisfied; the parameter mismatch between the MLs has a significant influence on their synchronization quality, but no obvious influence on their synchronization quality of the matched master-slave laser pairs, which further demonstrates the validity and feasibility of the chaos multiplexing scheme. More importantly, in this paper, the multiplexed chaotic signals in the time and frequency domains in terms of autocorrelation function and power spectrum are analyzed, demonstrating that the present system could provide higher security than the single external-cavity semiconductor laser.