Yeah, like Dan said, carry the newborn. Much nicer to you, a nb needs the comfort of a mothers scent more and your toddler will want to observe more. It's not like you're going to be ignoring your firstborn. In the end, they would rather a mummy without backache who is able to play when you get home that one who will carry them and be too tired and sore! You'll be great! Don't sweat it before it happens!