I'm currently nursing a 31 month old. She has always LOVED nursing and would nurse nearly nonstop all day if I let her, which I did at first! Because of this I don't think she would make a good tandem nurser. We've cut back quite a bit on nursing (0 to 4x/day now) and I do plan on weaning before the baby is born. My original goal was to nurse to a year and then see what happened, and it just kept happening. I'll be satisfied with a 3-year run!
*Actually* weaning, on the other hand... well that will be a whole new thread in a few months I'm sure!