In a large saucepan, combine rhubarb, strawberries and lemon juice. Cover and cook over medium heat about 5 minutes. Combine sugar and cornstarch; stir into saucepan. Bring to a boil, stirring constantly 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 13X9X2 inch baking dish. Carefully spread filling on top. Drop remaining batter by tablespoons over filling.
For topping melt butter in a saucepan over low heat. Remove from heat; stir in flour and sugar until mixture resembles coarse crumbs. Sprinkle over batter. Lay foil on lower rack to catch any juicy fruit spillover. Place coffee cake on middle rack. Bake at 350 degrees for 40-45 minutes. Cool in pan. Cut into squares.