Why are the King and the Duke forced to hatch an evil plot as they whisper together in the wigwam on the raft?

The Duke and the King are a couple of con-artists, hiding out on Jim and Huck's raft to escape the last group of suckers they ripped-off. But these greedy charlatans haven't learned their lesson; they're still shamelessly scheming, thinking about what their next dastardly move will be. They take over the raft's wigwam, which had previously provided shelter for Jim and Huck. While there, they discuss their plans. None of their cons have brought in much money since they arrived, so they start to think of a better idea, one that's practically guaranteed to line their pockets. Despite Huck's tall story about Jim's being his slave, it takes a lot to con a con artist, and the Duke and the King strongly suspect that Jim's a runaway with a large bounty on his head. There's only thing for it—the two hucksters are going to grab Jim when the time is right and turn him in for the reward money.
But of course they mustn't let on. That's why they whisper to each other ever so quietly in the relative privacy of the wigwam. Jim and Huck are uneasy at the sound of all the hushed conversation emanating from the little tent, and with good reason too.