Heat oven to 375°F. In large bowl, mix strawberries, blackberries and 1/4 cup of the sugar; set aside.
Separate dough into 10 biscuits; press or roll to form 2 1/2-inch rounds. With 1-inch round cutter, cut hole in center of each round. Dip all sides of biscuits and centers into melted butter, then into remaining 1/4 cup sugar. Place on ungreased cookie sheet.
Bake 12 to 14 minutes or until golden brown. Meanwhile, in small bowl, beat whipping cream with electric mixer on high speed until soft peaks form.
To serve, place 1 warm doughnut on each of 5 serving plates; fill with berries and whipped cream. Top with remaining doughnuts.