I've tandem nursed twice - once for 25 months and once for 36 months. Nursing while pg was actually harder than tandem nursing. I did have to make some adjustments the first time around - I had to assign sides, with my older nursling not allowed both sides though the youngest could go to the second side if needed. (Also, I have one high producing and one lower producing side, so that made it easy to figure out who got what side. I only assigned sides due to the baby getting too much foremilk). The next time around, I did not need to assign sides.
As for night weaning, I thought tandem night nursing would be too hard on me. I have rough pregnancies and also wanted to get my fertility back. So I nightweaned my first. I started at 18 months and it took until 22 months since I was doing it as gently as possible. He still woke up every single night asking to nurse. I figured that as long as I kept comforting him in ways other than nursing that it would eventually work. Nope. It took longer to get him back to sleep without nursing and he woke every single night. I gave in - more than a year later - when his waking was waking up the second child who was a newborn then. So, the next time around, I did not nightwean. (Also, my middle child dayweaned herself at 2 when she dropped her nap. She could only nursing lying down from 4 months on. So she had to bed, two wake-ups, and first thing in the morning. Nightweaning her would have dropped her down too much). Tandem night nursing really wasn't that bad. There were some nights that were rough - but that was only when there was illness or teething, which would have been rough anyway! I did feel like a rotisserie chicken at times, but it went well. My second child was allowed to nightwean herself, which she did right around her 5th b-day. So, that's just over 2 years of tandem night nursing that pair and it went well. (WIth the other pair, I gave in when the oldest had just turned 3 and I did gently nightwean him when he was ready - which was 4.5 years old). Nightweaning does work for a lot of people. But my oldest was obviously not ready. I likely could have more successfully nightweaned my second, but it would have cut her nursing down from 4 to 2 session. And I think it really helped her with the transition of a new baby.