Lemiis my biggest concern is for my dd who will be just 3. The impact on her will be just massive. I don't really know how is best to make it easy on her.
A couple of things I have thought about:-
Try and plan things with dd and just one baby or the other to try and take away the 'two of them, one of her' feeling.
Try somehow to do things with dd on her own. Maybe just once a week- quick trip to the park or to a friend's house for a cuppa.
Take all the help I can get from family members! Use it to get them to hold the babies while I play with dd. Babies don't really care who is holding them when they're little.
When explaining the pregnancy to dd, we'll try and pitch it but as mummy and daddy having two babies, but all three of us having two babies- Ie they are her babies as well and she is theirs.
Don't always put babies needs first just because they are small and the are two of them. Newborn needs are simple but toddler needs are more emotional and therefore babies might sometimes have to wait.
I'm saying all this and in reality, I haven't got a fucking clue! This is all guess work. It's also a bit sad that the babies will never have as much of my attention as dd did, but I guess that's always true of younger siblings.