A mother under normal circumstances should be able to have enough milk for her twin babies. You are doing the right thing by expressing breast milk and giving the same to your babies. Express your breast milk in a clean sterile container and this milk can be kept at the room temperature safely for six hours and in the referigerator but not in the freezer for 24 hours. If the babies are gaining weight adequately there is no need for you to supplement with top feeds now may be till they are 4 months old when supplemental semi-solid cereals can be gradually introduced.