Heat a skillet over medium-high heat. Once hot, brown the guanciale until crispy and golden, about 2 minutes, tossing occasionally for even browning. Transfer the guanciale to a paper towel-lined plate and let cool. In a bowl, mix together the guanciale, eggs, grated pecorino Romano, 2 tablespoons mayonnaise and a couple of grindings of pepper. Arrange the bread slices on a cutting board or flat work surface. Spread the remaining mayonnaise on each slice. Divide the egg mixture between two slices, then close the sandwiches with the remaining bread, mayonnaise-side down. Cut both sandwiches in half either lengthwise or on the diagonal to make 4 tramezzini.