I agree that showing photos of tandem nursing helps the older one really get the idea, and understand that it doesn't mean no more nursing for them. I also talked a lot about how the new baby can only have nur-nur, and has to do it many many times all day and night in order to grow big and strong so she can play with her big sister. And we told her that since she is almost 3 and a half she can eat yummy things like apples and pretzles and chicken, but baby can't eat any of that stuff so baby will have to nur-nur more than she will.
So far its going really well for us, though my older dd is a year older than yours and I do think that helps. I can't nurse them both at the same time as often as my older dd would like, but she does like to help put my nipple into baby's mouth--Im sure you can imagine just how "helpful" that truly is (not) haha. But it helps her feel involved. And when the baby cries, dd says "mama, you've gotta nur-nur our baby!"
Married 12/08 to Chilean DH and mama to DD 2/2/10. We're a bilingual home and we and
Expecting #2 in late June!