frosty, I can't really tell what's been happening with you - I'd be wondering if you managed to start expressing from the very start of your dc1's stay in scbu and what happened in the early days with dc2. Did you keep dc2 with you, skin to skin, day and night - that would help a lot. I don't understand about the babies 'not sucking much' - I'd want to know why this was happening, and when it started.
I don't think your issue was with delayed let down. If the underlying dificulties with breastmilk removal aren't resolved, this is what causes a lack of production - get the production sorted, and things happen! Delayed let down can happen as an occasional, temporary thing, but I don't know of any underlying condition that causes it to be ever-present.
Some women have a hard time getting bf going, and they need all the help they can get at first. I don't think your babies can have been feeding well - if they were, they would have thrived.
This time round, it will help to have your baby with you all the time, on you. If there's a problem, start expressing straight away. Find someone who is truly expert at assessing positioning and attachment to check the baby is removing the milk ok, and watch for normal poo'ing from the start, too, as another check.
Hope this helps.