hmmmz...are they peeing and passing motion as per normal? if yes, they should be latching well, and perhaps just going through a growth spurt, and are constantly hungry. and it could be even more overwhelming for you since you have twins. i remembered being so tired when my girl was going through a growth spurt and constantly latching..it's like i do nothing the whole day but just latched. pump volume doesn't mean the amount of milk you have, and since you're already latching, and for twins, i'm not surprised pump vol is low.
jiayou mummy!