You can cook two pounds of spaghetti, fusilli, ziti—or any other type of pasta—in a single pot.
Cook 2 pounds of pasta in a 6-quart Dutch oven in the same 4 quarts of boiling water and 1 tablespoon of salt you would use for 1 pound of pasta. (It’s the ratio of salt to water, not salt to pasta, that’s key for seasoning.)
Be sure to constantly stir the water after adding the pasta to keep it from sticking to itself (once the water is boiling, the bubbles will do the job).
This cooking water will be extra starchy, so add it in small increments when using it to adjust the consistency of a sauce, and switch to hot water if you find that the sauce isn’t thinning properly.