Mix the chopped rosemary, garlic, lemon zest, lemon juice, 1 tablespoon olive oil, 2 teaspoons salt and 1/2 teaspoon pepper together in a small bowl to make a paste.
Place the chickens on a sheet pan, skin side up, and loosen the skin from the meat with your fingers. Place 1/2 of the paste under the skin of each chicken. Rub any remaining paste on the outside and underside of the chickens.
Turn the chicken skin side down and scatter the lemon slices and sprigs of rosemary over each chicken. Season with salt and pepper. Roll each chicken up, cover with plastic wrap and refrigerate for at least 1 hour.
Heat a grill with coals. Spread the coals out in 1 dense layer and brush the grill with oil. Unroll the chickens, place them on the grill and cook for 12 minutes on each side.