To make the topping, whisk together the flours, salt, sugar, and cinnamon in a large bowl. Cut the butter into a few large pieces and toss these in the bowl with dry ingredients. Use your fingertips and rub together the flour mixture, butter, oats, and coconut flakes in a large bowl until the mixture resembles very coarse breadcrumbs with a few larger lumps. . Leave the mixture in the fridge to firm up for 30 minutes while you're preparing the filling.
Cut the rhubarb into 1 inch lengths and put in a large bowl.
Peel, core, and use a mandolin or sharp knife to slice the apples into 1/8 inch thick then add them to the rhubarb.
Add sugar, lemon juice, and cornstarch to the mixture and toss gently to combine.
Preheat the oven to 375 degrees F. Butter a shallow 2-qt. baking dish.
Transfer the fruit to a large baking dish. Scatter the crumble topping evenly over the fruit. Don't pack it down.
Put the baking dish on the middle rack of the oven and bake for 30-35 minutes or until the crumble is golden on top and the fruit is soft and bubbling underneath.
Let the crumble cool for 10 minutes before serving with vanilla ice cream.