Place four 6 1/2-inch cast-iron skillets in the oven and preheat the oven to 425 degrees F.
Whisk the cornmeal, sugar, baking soda and salt in a large bowl. Add the Parmesan and Cheddar and toss lightly to mix.
Whisk the buttermilk, melted butter and eggs in a second bowl. Pour into the cornmeal mixture and mix until just combined.
Carefully remove the skillets from the oven. Divide the remaining 2 tablespoons butter among the skillets and let it melt. Swirl the skillets to coat them with the butter and divide out the batter. Bake the cornbreads until golden on the edges and a tester inserted in the center comes out clean. 14 to 15 minutes. Let cool on a rack.