Filling: In a med saucepan, combine apples, plums, sugar, water, corn starch, and salt.cook qt med heat for 10 minutes or until thickened. Set aside.
In a large bowl, sift flour, baking soda, baking powder. Cut in butter with pastry cutter. Stir in brown sugar and rolled oats. Press half of mixture into greased 13 x 9 inch baking pan. Pour fruit mixture on top; top with remaining flour mixture.
Bake at 350 degrees for 45 minutes, until top is slightly browned. Cool before cutting into bars.