Preheat oven to 190 C / Gas 5. Grease a 23cm baking dish.
Boil peaches and nectarines 1 fruit at a time in a pot until very soft, about 30 seconds each. Let cool until ready to handle. Peel, core and chop into bite-sized pieces.
Mix the fruit, sugar, cornflour and vanilla extract together in a large bowl. Pour into the prepared dish.
Combine flour, brown sugar, coconut oil, baking powder, cinnamon and salt in a bowl. Mix with hands or a fork until crumbly. Sprinkle evenly over the fruit mixture.
Bake in the preheated oven until top is browned and juice is bubbling, about 30 minutes. Let cool for at least 15 minutes before serving.