In a small bowl, toss together both cheeses. In a 12-inch skillet over medium, cook the guanciale, stirring occasionally, until browned and crisp, 4 to 6 minutes. Using a slotted spoon, transfer to a small bowl; set aside. Measure 1 tablespoon of the rendered fat into another small bowl; if you used pancetta and do not have 1 tablespoon rendered fat, make up the difference with olive oil. Pour off and discard any remaining fat in the skillet and reserve the skillet.

To the rendered fat, whisk in 2 tablespoons of the cheese mixture, the egg yolks and 1½ teaspoons pepper; set aside. To the skillet, add the spaghetti, placing the noodles parallel to each other in the center of the pan, then add 2 tablespoons of the pecorino-Parmesan mixture and ½ teaspoon each salt and pepper. Add 4½ cups water. Bring to a boil over medium-high and cook, uncovered and frequently moving the pasta about with tongs but keeping it submerged as much as possible, until just shy of al dente; when done, the pasta will no longer be fully submerged and there should be a fair amount of starchy liquid (at least 1 cup) in the pan. If the pasta is very underdone and the water already is quite reduced, add another ½ cup water and cook, stirring often, until just shy of al dente.

With the skillet still on medium-high, add about half of the remaining pecorino-Parmesan mixture and toss until fully melted. Add the remaining cheese mixture in 2 more additions, tossing until melted after each. The liquid will have formed a thin sauce that pools in the pan. Continue tossing the spaghetti until al dente and lightly sauced, with a small amount of creamy sauce (about ¼ cup) pooled in the skillet; the timing will vary based on how much liquid was in the pan before the first addition of cheese. But at this point, cooking should take a couple of minutes at most.

Remove the pan from the heat. Add 1 tablespoon of the sauce in the skillet to the yolk mixture and whisk to combine. Now add the yolk mixture to the pasta; toss until evenly distributed and the sauce clings to the noodles. Toss in the guanciale. Taste and season with salt. Serve sprinkled with additional pepper, if desired.