In a mixing bowl, combine the strawberries, blueberries, sugar and lemon juice together. Using the back of a fork, lightly mash 1/4 of the strawberries. Split the shortcakes in half. Spread the bottom of each shortcake with 2 tablespoons of the lemon curd. Spoon the fruit mixture over the lemon curd, along with any juices. Top the fruit with the tops of the shortcakes. Garnish the shortcakes with the whipped cream, powdered sugar and fresh mint sprigs.