Tough one Lulu but I would carry the newborn and have Iz in the pram. Iz is at the stage where she wants to look around and be able to touch and play and probably wouldn't even notice Ned being carried. So I would alternate between them both being in the double pram and Iz being in the pram and Ned being carried.
It's so hard knowing what to do so that you don't look like you are favouring one or the other. You are going to do an amazing job Lulu.