Step 1Rub oil over surface and inside of chickens. Combine seasoning salt and lemon pepper in small dish. Sprinkle chickens inside and out. (Can be done several hours ahead or overnight and refrigerated. Before roasting, let stand at room temperature 30 minutes.)
Step 2Place chickens, breast side down, on rack set in shallow roasting pan or jellyroll pan (be sure to space chickens at least 2 inches apart). Place shallots and tarragon, divided evenly, into cavities. Tuck wings under; tie legs together.
Step 3Roast at 425 degrees on lower oven rack until lightly browned, about 30 minutes. Remove from oven. Use large meat fork inserted into cavity to turn chickens breast side up. Roast until well browned and meat thermometer inserted between thigh and leg registers 180 degrees (or insert tip of paring knife to be sure juices run clear), about 25 minutes more, depending on size of chickens.
Step 4To serve, remove shallots and tarragon and discard. Cut chickens in half, removing backbones with kitchen shears. Transfer chickens to platter; garnish with tarragon. Serve hot or at room temperature.