Heat a gas grill to high. Using sharp kitchen scissors or poultry shears, remove the backbone of each hen by cutting on either side of it through the ribs. Turn the hens skin side up on the cutting board and press on the breastbone to flatten. Rinse each hen and pat dry. Spread the hens out flat on a large shallow baking sheet and sprinkle them on both sides with salt and pepper. Bend the wingtips 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 teaspoons each of the thyme and rosemary. Brush the hens liberally on both sides with the seasoned oil then add more pepper.
Grill the hens skin side down, uncovered, until the skin develops nice grill marks (rotate the birds a quarter turn after first 3 minutes to create a crosshatch pattern). Turn the hens and sear them on the second side, again uncovered, about 4 minutes.
Sprinkle the skin side with the reserved herbs and salt, close the lid, and reduce the heat to medium. Cook until the hens are tender throughout and the skin is crisp and very golden; an instant read thermometer inserted in the thickest part of the thigh should read 180°F, 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.