Combine first 4 ingredients; stir in strawberries, rhubarb, and lemon juice. Let stand 20 minutes.
Roll 1 piecrust to an 1/8-inch thickness on a lightly floured surface. Fit into a 9-inch pieplate according to package directions; fold edges under, and crimp. Chill 10 minutes.
Spoon strawberry mixture into piecrust; dot with butter.
Roll remaining piecrust to an 1/8-inch thickness; cut into 1/2-inch strips. Arrange in a lattice design over filling. Sprinkle with 2 teaspoons sugar.
Bake at 425° for 15 minutes; reduce heat to 350°, and bake 30 more minutes or until crust browns.