this is a great topic, so I started a discussion for it!
Have you read the book; Adventures in Tandem Nursing by Hillary Flower? The book discusses breastfeeding through pregnancy as well as tandem breastfeeding.
For many moms, as the milk changes to colostrum and decreases in quantity, it becomes uncomfortable to nurse during pregnancy. A whole bag of mixed feelings can come along with that. It can be really challenging. The physical discomfort usually disappears as soon as the milk 'comes in' again after the birth of the newborn. Some moms feel so 'touched out' from what seems like nursing all day! Some moms really appreciate the time with the toddler nursing to reconnect, calm tantrums, get to sleep, etc...
Some moms experience negative feelings toward their toddler wanting to nurse after the baby is born and 'protective' feelings about the newborn.
It can be really important to love and care for ourselves through these feelings; accept them and work through them toward a solution that fits our family. Support from other mothers who have experienced this can be realy helpful.
One of my favorite nursing stories is when my older two kids were nursing at the same time. The oldest was 2 at the time and my daughter was a newborn. The two year old picked up his head and said; "I love my baby sister" then smiled at her and went back to nursing. He would also play with her fingers, rub her back or head, and just cuddle her while they were both nursing.