Duke and Larry are Jubal's handymen. They also happen to be handy characters—good for rounding out opinions on Mike.
Duke originally has a problem with Mike and his Martian customs, particularly the Martian tradition of cannibalism. After all, things are different in Kansas, where he's from: "Okay, so I'm from Kansas. Never was any cannibalism in Kansas" (13.4). But after talking with Jubal and realizing that the world (sadly) doesn't revolve around Kansas, Duke decides that what Mike eats is up to him, and he even becomes his water brother.
Later in the novel, Duke joins Mike's church. He serves as the in-house deputy fire warden, sanitary and safety inspector, plumber, electrician… you get the idea. That way, no one from outside the Nest has to come into the church. Clever. He and Mike even joke about Duke's previous squeamishness to Mike's Martian culture, showing the bonding power of laughter.
At the end of the story, Duke cooks Mike into a broth and eats it with Jubal, showing how far he's come since his initial rejection of Mike's customs. But he doesn't do it without complaining: "Needs a little salt" (38.53).
Larry… well, Larry hangs around. He fixes things, does the occasional chore, and joins the Church of All Worlds. That's about it for Larry. Why do you think Heinlein decided to keep him in the story?