In a large bowl combine flour, sugar, butter and coconut oil. Mix with hands. When the ingredients come together, add cold water and quickly work into a soft dough. Flatten, wrap in cling film and place in fridge for 30 to 60 minutes.
Place rhubarb in a large pot; add sugar, orange juice and zest, jam, vanilla sugar and cinnamon. Mix and bring to the boil on medium-high heat. Cover, reduce heat and simmer for 5 to 7 minutes until rhubarb is soft.
Pass rhubarb mixture through a sieve into a clean bowl. Pour the syrup back in the pot and boil until thickened, about 20 minutes. Add rhubarb pieces back in the pot with thickened syrup and stir gently so the rhubarb doesn't fall apart.
Preheat oven to 190 C / Gas 5. Lightly grease a tart tin.
Divide the cold pastry into 2 parts - one smaller and one larger part. Roll out the larger part on a lightly floured surface. Lay it in the prepared tin and press gently to cover the bottom and sides of the tin. Sprinkle desiccated coconut over the pastry and press gently into the dough. Bake for 10 minutes in preheated oven.
Roll out the smaller part of the pastry and cut into long strips.
Pour rhubarb with syrup into baked pastry base and cover with the strips of unbaked dough. Bake for 55 minutes. Once cooled, dust with icing sugar.