Dissolve gelatin powder in boiling water. Add cold water. Cool slightly. Measure two cups, pour into heart molds. Place in refrigerator and chill until slightly thickened, or about one hour. Return remaining gelatin to refrigerator to chill slightly. Fold in whipped topping. Spoon over firm gelatin in molds. Chill until firm, or three hours. Unmold by running smooth knife or spatula around top edge. Dip in hot water five seconds. If necessary, place in microwave set on defrost for 30 to 60 seconds. Unmold each heart mold onto serving platter.