Combine sour cream, sugar and 2 cups of shredded coconut in a bowl. Cover and set in refrigerator, preferrably the day before you make the cake.
Make white cake according to box directions and bake in 2 layers.
Once cakes have cooled split into 2 layers using a knife or cake leveler.
Remove 1 cup of the sour cream, sugar, coconut mixture and stir into the Cool Whip - set in the refrigerator.
Divide the remaining sour cream, sugar, coconut mixture between each of the layers - placing a layer down spread some of the mixture on, add another layer and continue until all layers have been used.
Spread the Cool Whip mixture over the entire cake, sprinkle with remaining shredded coconut and refrigirate.