* swings & bouncers - it was really nice to have a place to put them when I was eating, for a place to nap, or to prop them up after eating so they wouldn't get reflux. 2 bouncers are easy to bring with you when visiting family in the early days, so you have a safe place to put them or let them nap.
*carriers - for babywearing, actually had more than 2 so we could each wear one. Moby wrap let me double wear them for months, though
*dressers - the clothes storage gets out of control quickly with just 1!
* double stroller - I used the snap n go for the first 6 mos or so, and by then had a better idea of what kind of real stroller to get, what my needs were
* high chairs
* tub seats - we used our for a long time, so they were a big deal. You can't be hanging on to one baby while you're washing the other, so the seats kept them stable & safe
* potty seats
* crib - in the beginning we just used it for the mobile, so I could leave them entertained for 10 mins for a shower or whatever. Also an easy place to get them dressed, assembly line style. They did have a few naps in there, too. We cosleep, so later we ended up side-carring the crib.
* activity/gym mat
* ez2nurse pillow
* baby bathtub - You bathe them one at a time as infants. Our FP one was big enough that later on I could put both babies in it at once, one sitting up and one reclining. That was great when they were little, before they were sturdy enough for the tub seats.
* diaper changing pad/table
* superyard - we did use this at times to give them a protected place to play, so I wouldn't worry about their big sister or our dog bothering them while I, say, cooked a meal. When they were mobile, it came in handy for babyproofing. Also great for putting around our Christmas tree, or for putting on top of a tarp when you're camping and they're not walking. We still have it out and they are 2! You might even need a 2nd one, depending on your house.
That's all I can remember as far as the big stuff goes! I left off the obvious stuff, like clothes, diapers, a pump.