Cut pancetta into 1/3-inch dice, then cook in a deep 12-inch heavy skillet over moderate heat, stirring, until fat begins to render, 1 to 2 minutes. Add onion and cook, stirring occasionally, until onion is golden, about 10 minutes. Add wine and boil until reduced by half, 1 to 2 minutes.
Cook spaghetti in a 6 to 8-quart pot of boiling salted water until al dente.
While pasta is cooking, whisk together eggs, Parmigiano-Reggiano, Pecorino Romano (1/3 cup), 1 teaspoon coarsely ground black pepper, and 1/4 teaspoon salt in a small bowl.
Drain spaghetti in a colander and add to onion mixture, then toss with tongs over moderate heat until coated. Remove from heat and add egg mixture, tossing to combine. Serve immediately.
*Cook's note: The eggs in this recipe will not be fully cooked, which may be of concern if there is a problem with salmonella in your area.