If you prefer using the sorbet function on your blender, I would a bit of vegetable milk or yoghurt and press "sorbet". If you don't have a sorbet function, or you are making the two ingredient version, transfer the frozen mango and raspberry to your blender. You can still add some vegetable milk or yoghurt. This smoothens the ice cream a little bit and lowers the temperature of the frozen fruit, making the blending go a little bit faster. But you should also be able to make it without.