K.G.
I got pregnant with my second when my first was 11 months old. She actually stopped herself nursing at around 17 months, because the milk tastes different when you are pregnant. I planned on tandem nursing, I read all the books, spoke with La Leche and other mothers.
My breast were very tender and it was very painful for my first to nurse when I was pregnant, so I felt guilty that she lost that extra year when she weaned herself.
I would make all the plans, then wait and see what happens for you once the baby is born. Whatever you decide, remember, the best gift you can give your first child is a sibling. so let go of the guilt, do the best you can, and see what happens for you.
Congrats and good luck!
Also, be prepared. An older woman once told me (regarding children) one is like none, and two is like ten!