Split each tail lengthwise (heavy scissors help) to bottom. Open and lift tail on top of shell leaving bottom connected. Brush with butter. Arrange in a shallow glass dish. Microwave (high), covered, 9 to 11 minutes. Rotate dish 1/4 turn halfway through cooking. Meat will be firm and translucent; shells will be pink. Rest 5 minutes. Serve with butter and lemon juice.