I have had my second baby already but thought I'd share my experience.
I was breastfeeding my baby when I got pregnant and although I hadn't actually planned to stop at any particular point, he just self-weaned one day out of the blue when I was 3 months pregnant. Totally took me by surprise but he just decided that he didn't want it one evening! The next morning he sucked for a coupe of seconds and the next morning but that was it. Very strange but it was just how I wanted it to happen. I dreaded having to "cut him off", and lucky for me he made the decision himself. Now I have my newborn (11 weeks), I really think it would have been a huge strain having to nurse two children at the same time - that's just my personal opinion. In the beginning you are nursing soooooo much with a newborn, and that takes so much out of you, physically and emotionally. Funny, even though Toby only stopped nuring 6 months before Gus was born, he didn't even worry about it at all - he just accepts that Gus is having his "milkies". He's actually very sweet and understanding about it - almost like he's moved over and said "I had my bit, now it's Gus's turn!"