I think that the author is making a statement with these two characters. I think he is trying to point to how willing people are to be really evil in pursuit of their own gain.
If you think about the things these two men do, they really are quite horrible, even though the two of them just come across as a couple of clowns. The two of them are willing to swindle people at a church service. They are willing to scam a family that has just had a member pass away. And they are willing to betray Jim and sell him back into slavery.
So I think Twain is remarking on how selfish and evil people can be.