Place lime juice and 1 cup each blackberries and blueberries in an 8-inch square glass dish. Mash berry mixture using the back of a spoon.
Layer half of cake cubes evenly in each of 4 dessert glasses. Top evenly with half of berry mixture and half of reserved topping mixture. Repeat layers with remaining half of cake cubes, berry mixture, and topping mixture. Top each trifle with 1 tablespoon each remaining blackberries and blueberries.
Shortcut kitchen tip
Mashing the berries releases their juice, which adds flavor and moisture to the cake. To easily mash the berries, place them in an 8-inch square glass dish or other container with a wide, flat bottom, and use the back of a spoon.