Theodore and Amos used to be part of the Stillwell Gang but were kicked out because Amos accidentally shot the leader, Frank Stillwell, in the leg. They now call themselves the Halve Knives. They are a bumbling duo of robbers that Homer McCoy said, "They couldn't steal candy from a baby without coming out of the short end".
After Bobby, Clovis, and Celia Bradley find a gold nugget, Theodore and Amos try to steal it but get caught by Homer McCoy who sentences them to be hung. But they are getting their chance to it, for when John Whintle, the children's second uncle, is going to get them (for it's all he wants) away from Russell Donovan, he makes a deal with them to take it so the children can stay with Russell, and to make things calm.
He tells them, "It's not actually stealing, it's our gold we're giving it to you" But things weren't going well for Frank also knows about it and plans to steal it. But the town finds out and he and his gang escape with Celia. He tells Theodore and Amos they can have it, but they accidentally destroy it with a pack of dynamite. In the end, they decide to quit the robbing business and live with Russell and his new family.
A wanted poster for Theodore and Amos can be seen in the queue for Big Thunder Mountain Railroad in the Magic Kingdom at Walt Disney World. It identifies them as having attempted to rob the T.W. Silver Mine in Virginia City, Nevada. They went about this by posing as, "metallurgists" in-order to take metal for claimed examination while in-reality stealing it.
T.W. Bullion sent the wanted poster to their relative, Barnabas T. Bullion, of the Big Thunder Mining Company, warning him of them. However, as it would turn out the pair only stole a few pieces of lead which they believed to be silver with him stating them as being completely incompetent and only having fouled up their operations.
Theodore and Amos as they appear in The Apple Dumpling Rides Again