Preheat oven to 180 degrees C. Grease 2 large loaf tins.
In a bowl, whisk together 2 3/4 cups of flour, 2 teaspoons of cinnamon, the bicarb soda and salt until thoroughly combined. In a large mixing bowl, beat the melted butter with 2 cups of sugar until smooth and mix in the eggs one at a time, followed by the Greek yogurt and vanilla essence. Stir in the bananas, apples and chopped walnuts until well combined; do not over mix. There should be small chunks of banana visible in the batter. Pour half the batter into each prepared loaf tin.
Mix the brown sugar, 1/2 cup of white sugar, 1 tablespoon of cinnamon, 1/4 cup of flour and 3 tablespoons of softened butter together in a bowl until the mixture resembles coarse crumbs. Sprinkle half the crumb topping over each loaf. Garnish each loaf with banana and apple slices.
Bake in the preheated oven for 30 minutes then remove and cover each pan with aluminium foil. Return to oven and bake an additional 15 or 20 minutes or until a toothpick inserted into the centre of the loaves comes out clean. Allow to cool before serving.