Correct me if Im wrong, but as twins are usually alot smaller than singleton babies, then I would think that taking less formula would be normal. A baby who was born 5-6lbs would probably only be taking 2-3 ounces in the early days and weeks.
As a general rule if you multiply your baby's weight by 2.5, then that is approximatly how many ounces of milk they need in a 24 hour period. So for a 6lb baby they would need roughly 15oz of milk per day. Split that into 6-8 feeds per day, works out at 2-2.5oz per feed. xxx