Peel the shrimp, reserving the shells. Carefully devein the shrimp and coarsely chop. Coarsely chop the scallops and combine with the shrimp and 1/4 cup water in the bowl of a food processor fitted with the steel blade. Pulse on and off to finely chop but don't create a puree. Add another tablespoon of water if the mixture becomes sticky. Cover, and set aside in the refrigerator while you make the broth.

Start the broth, or fumet, by heating the olive oil in a large straight-sided saute pan. Add the reserved shrimp shells, onions, garlic, and hot pepper. Saute over moderate heat for about 10 minutes until the onions have softened and the shells have turned deep pink. Add the wine and reduce until you have 1/4 to 1/3 cup left in the pan. Add 4 cups water and boil vigorously to reduce until you have about 3 cups of liquid left. Strain the fumet through a fine sieve into another large saute pan. Add the tomato and bring to a boil. Taste and add salt, if necessary. Set aside over very low heat and simmer gently while you cook the pasta.

Bring at least 5 quarts of water to a boil in a large pot. Generously salt the water and drop in the spaghettini. Cook, stirring often, until the pasta is almost al dente, about 4 minutes. Drain the pasta and transfer to the fumet. Fold in the chopped shrimp and scallops along with the butter and parsley. Simmer just until the spaghettini is al dente and the seafood is cooked through, 1 to 2 minutes. Serve immediately.