Combine rhubarb, sugar, and 1/3 cup water in saucepan; bring to a boil. Reduce heat; cover. Simmer 5 minutes. Add strawberries; cook until strawberries are tender, 2-3 minutes.

Blend together cornstarch and 2 T water; add to rhubarb mixture. Cook, stirring constantly, until mixture thickens and boils. Remove from heat. Chill in refrigerator until serving time. Yield 3 cups.