Combine cheeses in a small bowl; set aside. Place ground beef in a medium bowl. Sprinkle with Worcestershire, salt and pepper. Mix gently, just until combined. Don’t over-mix, or burgers will be tough! Divide beef into 12 equal portions. Flatten each into a thin patty about 4-inches in diameter. Place 1½ tablespoons cheese in center of 6 patties, leaving a border around edges. Cover each with another patty. Pinch edges to seal, then pat top of each patty to flatten. Gently pat sides of stacked patties to make a uniform-looking burger. Refrigerate stuffed patties for at least 10 minutes and up to 4 hours. Preheat grill to medium-high; clean and oil grates. Grill burgers until cooked through, about 4 minutes per side. Serve on buns with your favorite toppings and condiments.