New here, but I came across your post and can offer some encouragement. I have 5 children, the last two being 6 month old twins. I have nursed during all of my pregnancies (except the first, of course). Our children are all 19mo-25mo apart, so it sounds like we may have similar spacing? Anyhow, my experience was that they all nursed for the first 4-5 months of a pregnancy (at which point, for some reason, my milk changes so much that they no longer want to nurse!), so I cannot speak for nursing through an entire pregnancy, but I did spend the first 4-5 months of being pregnant with twins and nursing.
It was exhausing, obviously, but my midwife was totally behind it and then when I had to switch to doctors, never had a problem from any of them. WHen I was pregnant with my second, I had a doctor that was very angry when I told him I was still nursing (which I never quite understood) and basically told me that I would become anemic and never replenish my iron levels. Fortunalty, I had done my research and knew this was not true. Needless to say, we have a new doctor!
I would say if you drink enough fluids, keep up on your prenatals (I was bad about this) and eat a lot of protein, which your body may already be telling you to (I remember having to eat 2 eggs as soon as I woke), you could keep it up. The only thing I was warned about was making sure I had enough iron, which was not a problem.
That's great that you are keeping your 1 year old close at night. Our 2 year old still occasionally crawls in bed with us in the middle of the night, and it is sweet to see her and her baby brothers all together in the morning. You are in for a real treat! Twins are a true blessing!
Hope this was helpful, if only to know you are not the only one!