Preheat oven to broil with 1 rack on bottom of oven and 1 rack about 4 1/2 inches from heating element. In a food processor, whirl garlic until finely chopped. Add butter, garlic, and cayenne and whirl until blended. Halve the bread lengthwise and spread with garlic butter. Broil bread, cut side up on a baking sheet, on bottom rack until butter is sizzling, about 2 minutes.
In a medium bowl, toss together the cheeses. Remove bread from oven and sprinkle with cheese mixture. Broil bread on top oven rack until cheese has melted and is just beginning to turn golden, about 1 1/2 minutes. Transfer bread to a cutting board and cut into 1 1/2-inch-wide slices. 12 servings.