Combine cake mix and pudding mix in a large mixing bowl. Pour enough milk into the reserved pineapple juice to make 1 cup of liquid; add to cake and pudding mixture. Pour in vegetable oil. Mix on low speed, adding 1 egg at a time until blended. Increase speed to medium; mix until thoroughly combined and smooth, about 2 minutes. Pour batter over pineapples and cherries in the pan.