We're planning to start trying for #2 this fall - and since I'm a planner, I'm already starting my 'planning.' haha
So I know we'd need a double stroller. Right now have the Kolcraft Contours Tandem II and the Baby Jogger City Select on my list to check out. Any other good ones? I don't want one that one child is UNDER the other like I've seen some do.
Since DS is only 10 mo right now, I'm sure he's still going to be in his crib. We'd probably move his dresser/changer to the new nursery for #2. So we'd need a new dresser for DS. And we'd need another crib.
The room we're planning on using for the new nursery is currently a bright tangerine orange, and I'm leaving it that color. I love it, just such a happy color. So I'm considering neutral themes that can coordinate with that color. I've seen some whale and owl prints that might work.
Anything else I should be looking into? Thanks!
Re: What do you need for 2 that you don't have for 1?
For us it was double stroller, crib or toddler bed (DD we put into a toddler bed, but I don't think DS will be ready when #3 gets here so we're getting another crib).
We also picked up some new nipples for the bottles and as DS was born in a different season, and obviously a different gender, we needed some new clothes.
After DS was born we also had to get another convertible carseat, but that isn't something you'll have to worry about for a while.
Good luck with #2!
For us it was a crib and mattress,inexpensive changing table (ds had a dresser and topper), swing from my sister (i hated the travel one we had for DS) new nipples for the bottles, pump parts if needed, new storage bags for breast milk, girl stuff
, new pacifiers, diapers, WIPES WIPES WIPES
two in diapers calls for a lot of wipes, esp in the newborn stage. I got a diaper caddy so I could bring stuff from room to room so I could keep an eye on DS while changing the baby, diaper creams, nipple cream, breast pads, pads, second themometer and tylenol (one for each kid just in case one was sick I didnt want to get the other sick if I could help it--and it came in handy around a few months old). a second camera for our video monitor, second sound machine, BJ city select (GREAT stroller) and accessories, car seat--later on of course, and the best purchase, a minivan 
I hear you about planning, I am ttc in the fall for #3 and though the list is shorter, I have already made a mental list of things I need to replace, or buy (or borrow if I can!) GL to you!
I agree with PP and all the things.. I don't have my 2u2 yet.. but almost..
I just had my infant carseat re-installed by someone from my county dept of public works who is trained by the carseat companies and safety experts. I told him that I just bought another one of last year's models of the Britax Marathon b/c I like it so much and want that for my new LO for when he's out of the the infant carseat (I don't like the re-designed Marathon that goes up to 65 lbs).. anyway.. he said I should have just bought a Graco Nautilus bc it is a top rated convertible carseat that turns into a booster and we would only have had to buy one thing instead of two. My older child could use that and the new baby would be able to use the Marathon when he is out of the infant seat. Oh well. Just thought I'd share the info from a professional
We're getting the Mountain Buggy +one double buggy. It's being released in August / September.
DS is getting a toddler bed and new mattress soon.
DC#2 will probably need some clothes. If it's a boy just some opposite season stuff, if it's a girl, well I reserve the right to go a little crazy....
That's it really.
Don't assume your DS will still need the crib by the time your #2 is born. My kids are 16 months apart and we did not get a second crib. We just used the bassinet in the PNP for the first few months with #2, and then moved #1 into a toddler bed and #1 into the crib.
Incidentally, #2 is now 20 months old and we just moved her into a toddler bed, and she's doing great. So one crib is definitely do-able.