Bring a large pot of water to a boil and season with salt. Once boiling, add the spaghetti and cook, stirring occasionally, until al dente. Drain, and reserve ¾ cup pasta cooking water.

Meanwhile, melt 2 tablespoons butter in a large skillet over medium heat. Add the cracked black pepper and cook, swirling pan, until toasted, about 1 minute.

Add ½ cup reserved pasta water to skillet and bring to a simmer. Add the spaghetti and remaining butter. Reduce heat to low and add Parmesan, stirring and tossing with tongs until melted. Remove pan from heat; add Pecorino, stirring and tossing until cheese melts and the sauce coats the pasta. (Add more pasta water if sauce seems dry.) Serve immediately.