Cut bananas in half crosswise, then lengthwise; set aside.
Melt the margarine in a large nonstick skillet over medium-high heat. Add brown sugar, apple juice concentrate, vanilla, cinnamon, and allspice, and cook for 30 seconds. Add banana pieces, and cook for 1 minute. Turn banana pieces over, and cook an additional minute.
Heat rum in a small saucepan. Pour over banana mixture. If desired, ignite rum with a long match; let flames die down. Stir gently.
Arrange 4 banana pieces on each of 2 dessert plates; spoon sauce over banana pieces, and top each serving with 1/2 cup frozen yogurt. Garnish with fresh mint sprigs, if desired. Serve immediately.