Things we've got or are planning to get:

Second hand and new baby clothes as we're having a girl and 8 bags of boys stuff just won't do lol
Couple of sets of new sheets.
Another car seat as DS is still in his.
Pram the will fit both of them. Note for anyone looking into this- I hated the idea of big chunky double pram but have found an amazing one called the Bootiq Duex stroller, very lightweight and folds up to size of single pram, plus comes with newborn foot muff, head support and storm cover and is really cheap!
Couple of new blankets etc as some of DS ones were worn out or stained.
2 extra baby gates second hand for $5 each as I know it will be harder to keep an eye on 2!

That's about it that we have 'needed' have bought some other stuff like a nursery manchester set worth $370 for $50 and some pretty girly teddy bears etc