Here are a couple old patents that should give you an idea of how the mold was made. As you have noted, there appear to be two patterns needed to make the outside (not counting the separate lid). Then there would be a core set inside, kind of like an inside mold, that would form the inside walls and prevent the iron from filling it up. The core would break up like the mold after casting leaving the teapot.
Patents 5,442 and 357,778 are examples. The core is letter C in the first, and letter S in the second.
Tom