The degradation of lignocellulosic materials requires the synergy of multiple enzymes. Synergistic fermentation would be an alternative to a complete enzyme system and consequentially enhance enzymatic production. The present study used cellulolytic fungus of Penicillium decumbens, Aspergillus niger C112, and Trichoderma reesei RutC-30 as the starting strains, to screen for the best combination and proportion of synergistic fermentation. The experiments of different combinations and inoculum ratios for synergistic fermentation were performed, and Box-Behnken designs were adopted to optimize the fermentation conditions. The best combination of strains was confirmed as T. reesei RutC-30 and P. decumbens with an 8:2 inoculum ratio. The maximum sugar concentration was 1.48 g/L with CMCase (17.53 U/mL) and FPase (4.78 U/mL) in this situation. Response surface methodology revealed the optimized parameters as a substrate concentration of 5.93% at 32.12 °C, and a fermentation time of 5.85 d with a predicted sugar concentration of 2.91 g/L. Under these conditions, a maximum sugar production of 2.96 g/L was achieved in verification experiments.