In a small 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 fresh lemon curd. Spoon a 1/4 of the fruit mixture on the lemon curd. Top the fruit with the remaining shortcakes. Garnish the shortcakes with the whipped cream, powdered sugar and fresh mint.