In a large saucepan, combine rhubarb and strawberries. Cover and cook over medium heat about 5 minutes. Add lemon juice. Combine sugar, cornstarch and salt; stir into saucepan. Bring to a boil, cook and stir for 4-5 minutes or until thickened. Remove from heat and set aside. In a large bowl, combine flour, sugar, baking powder, baking soda and salt. Cut in butter until mixture resembles coarse crumbs. Beat buttermilk, eggs and vanilla; stir into crumb mixture. Spread half of the batter evenly into a greased 13-inch by 9-inch baking dish. Carefully spread filling on top. Drop remaining batter by tablespoonfuls over filling. For topping, melt butter in a saucepan over low heat. Remove from heat; stir in flour, sugar and salt until mixture resembles coarse crumbs. Sprinkle over batter. Lay foil on lower rack to catch any juice fruit spillovers. Place coffee cake on middle rack; bake at 350° for 40-45 minutes. Cool on a wire rack. Cut in squares.