Heat a grill to high. Remove the backbone of each hen. Turn the hens skin side up and press on the breast bone to flatten. If desired you can cut through the breast to half the hens. Rinse each hen and dry.
Spread the hens out flat on a large shallow baking sheet and sprinkle both sides with salt and pepper. Bend the wings under the breasts to protect them from burning.
Pour 1/4 cup of the olive oil into a small bowl and add the garlic, a generous amount of pepper, and all but 1 teaspoon each of the thyme and rosemary. Brush the hens liberally on both sides with the seasoned oil and more pepper.
Grill the hens skin side down, uncovered, until the skin develops nice grill marks (rotate a quarter turn after first 3 minutes to create a crosshatch pattern). Turn the hens over and sear on the second side, again uncovered for about 4 minutes. Sprinkle the skin side with the reserved herbs and salt, reduce the heat and close the lid. If using a charcoal grill, move to side of coals to cook indirectly. Cook until the hens are tender and the skin is crisp and golden. A instant-read thermometer inserted into the thick part of the thigh should read 180 degrees, 10 to 15 minutes.
Put the hens on a large serving platter. Whisk together the remaining 1/4 cup oil and the lemon juice. Serve the hens drizzled with the lemon oil. Serve immediately.