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Anaerobes harbor some of the most efficient biological machinery for cellulose degradation, especially thermophilic bacteria, such as <i>Acetivibrio thermocellus</i> and <i>Thermoclostridium stercorarium</i>, which play a fundamental role in transferring lignocellulose into ethanol through consolidated bioprocessing (CBP). In this study, we compared activities of two cellulase systems under varying kinds of hemicellulose and cellulose. <i>A. thermocellus</i> was identified to contribute specifically to cellulose hydrolysis, whereas <i>T. stercorarium</i> contributes to hemicellulose hydrolysis. The two systems were assayed in various combinations to assess their synergistic effects using cellulose and corn stover as the substrates. Their maximum synergy degrees on cellulose and corn stover were, respectively, 1.26 and 1.87 at the ratio of 3:2. Furthermore, co-culture of these anaerobes on the mixture of cellulose and xylan increased ethanol concentration from 21.0 to 40.4 mM with a high cellulose/xylan-to-ethanol conversion rate of up to 20.7%, while the conversion rates of <i>T. stercorarium</i> and <i>A. thermocellus</i> monocultures were 19.3% and 15.2%. The reason is that <i>A. thermocellus</i> had the ability to rapidly degrade cellulose while <i>T. stercorarium</i> co-utilized both pentose and hexose, the metabolites of cellulose degradation, to produce ethanol. The synergistic effect of cellulase systems and metabolic pathways in <i>A. thermocellus</i> and <i>T. stercorarium</i> provides a novel strategy for the design, selection, and optimization of ethanol production from cellulosic biomass through CBP.