It is better to feed animals which age difference at different times. The old dog may have some food aggression toward your puppy. I would feed at different times.
First, you should feed them separate because the older dog might get agressive and try to bite the puppy. Second, puppies need a different kind of food and need to be fed more often than older dogs. I would feed them on different sides of the room or at different times.
By all means feed both of them at the same time, and preferably twice a day because the puppy will not be eating the same food, you should separate them at mealtime so one is not going after the other's food.