Francois makes this comment in response to Perrault, who has just observed that Spitz, the most dominant dog on the team, is "one devil, dat Spitz". Francois replies, "Dat Buck two devils", meaning that, in his opinion, Buck is an even more formidable opponent than Spitz. Although Buck, new to the pack, has held back and avoided confrontation as much as possible, "in the beiiter hatred between him and Spitz...betray(ing) no impatience, shunn(ing) all offensive acts", on the few occasions when the two have tangled, Buck has shown a ferocity and strength that makes Francois believe he will one day vanquish the lead dog and take his place.
Since he had killed Spits he was two devils.They said that Spits was a devil,but since Buck killed him he is two devils.
because that means (from my own insticts) that buck has the power to kill or tortur whoever he is fighting. You should like refer that since francois is french so it is hard to fine. GOOD LUCK ON FINDING TO TRUE MEANING