In a large enameled cast-iron casserole, melt 1 1/2 tablespoons of the butter in 1 tablespoon of the olive oil. Season the rabbits all over with salt and pepper. Add 1 rabbit to the casserole, breast side down, and cook over moderately high heat until the inner legs are golden, about 4 minutes. Turn the rabbit and brown each side until golden, about 8 minutes total. Transfer the rabbit to a platter. Discard the fat and wipe out the casserole with a paper towel. Repeat with another 1 1/2 tablespoons of butter, the remaining 1 tablespoon of olive oil and the second rabbit.