Set a large pot of water to boil.
Whisk eggs, yolks, Parmesan, Pecorino, and pepper in a large bowl until smooth.
Salt boiling water, then cook fettuccine until just al dente.
Reserve 1 cup pasta water. Drain the pasta.
Heat olive oil in a skillet over medium. Crisp pancetta until browned, 5–6 minutes.
Stir in garlic for 30 seconds. Remove the pan from heat.
Toss hot pasta with pancetta. Splash in half the reserved water.
Off heat, pour in egg mixture. Toss quickly until creamy, adding more water as needed. Season and serve.