Drain rhubarb and strawberries, reserving liquid from each. Cut rhubarb into ¼-inch pieces. Combine rhubarb and strawberry juices with enough water to make 2 ½ cups of liquid. In a pot on the stove, combine tapioca, salt and fruit liquid. Bring to a boil over medium heat, stirring constantly. Remove from heat and cool slightly. Gently stir in berries; let cool 15 minutes. Stir to mix well. Chill, uncovered, in the fridge for about 2 hours. Serve in individual dishes with whipped topping.