Nik was 21 months when Nate was born, and we tandem nursed until he turned 3. If I'm remembering correctly, for the first couple of weeks I tried to always feed the baby first. Sometimes, when the initial engorgement was bad, though, I would let Nik nurse to relieve the pain and then let Nate finish off the fatty hindmilk. Having a nursing toddler around is very handy for engorgement relief!!! After the first couple of weeks they often nursed at the same time, nursing from only one breast each during those feedings. Nik didn't nurse as often as Nate, though, so Nate nursed from both breasts during some feedings.
It takes a bit of shuffling, but unless your toddler is demanding to feed every time the baby feeds I think you will find that things quickly work themselves out and you find a rhythm. I think supply will meet demand in most cases. Just try to feed the baby first every time for a while, or let the toddler help with the engorgement if you have it and let baby have that fatty hindmilk.
I don't know how much help I was, but I wish you luck! It can be very challenging at times, but I think tandem nursing was a great help in keeping the new-baby jealousy at bay in our house. The boys, ages 6 & almost-8, love to look at pictures of them nursing at the same time. They are holding hands in most of those pictures. 
has a lot of great info. It might take your mind off these final (miserable) weeks. Not to beat a dead (nursing) horse, but I thought I'd throw it out there.
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