Clean the clams, discarding any with broken shells or any that won't close when you tap them. Place in a large bowl of very cold water with the cornmeal for about 30 minutes. Make sure to carefully mix stir the water to disperse the cornmeal. This will draw out the debris from the clams. Rinse and drain until the water is clear.
Bring a large pot of salted water to a boil and cook the spaghetti according to directions for al dente. (Cooked, but firm, not mushy)
Meanwhile, in a heavy-bottomed large sauté pan that has a fitted lid, heat the olive oil over a low heat. Add in the garlic, chorizo, peperoncini and chopped tomato and cook until the garlic is fragrant but not burnt.
Add in the clams and coat with the oil. Add in the shrimp and cover the pan. make sure to turn the shrimp over once pink on one side.
Add in the parsley and carefully mix to coat the clams once more.
Once the pasta is done, using tongs, add in the pasta to the pan. This will bring in some of the cooked pasta water which will thicken the sauce. Carefully toss to incorporate all of the ingredients, adding in additional cooked pasta water as needed so the pasta will not be dry. Serve immediately.