Prepare filling: In a large saucepan, simmer apples, cherries, and cider, stirring occasionally, until fruits are softened and plump, about 5 minutes. Reserve 2 tablespoons of liquid and drain remaining liquid. Let cool. Use kitchen shears to cut any large pieces of apple into bite-size pieces, or coarsely chop fruit. Combine with 2 tablespoons sugar, ¼ teaspoon cinnamon, and reserved cooking liquid.

Preheat the oven to 375°F. Butter a 9-by-13-inch baking dish or line with aluminum foil.

Whisk together oats, flour, ¾ cup brown sugar, 1 teaspoon cinnamon, baking soda, and salt, in a large bowl. Cut in butter with a pastry blender or 2 knives until you have pea sized crumbles. 

Press 3 ½ cups of the oat mixture into the bottom of the prepared dish (you’ll have about 6 cups.) Spread fruit filling on top, leaving a ¼-inch border on all sides. Sprinkle the remaining oat mixture on top.

Bake until golden brown, about 30 minutes. Let cool in baking dish on a wire rack completely before cutting. Cut into 24 bars. Bars can be stored in an airtight container for up to 3 days