-Rock and Plays!!! These are AWESOME and have probably been the most-used baby item we got! The twins hated sleeping in the double-bassinet pack&play and that thing was so bulky so we quickly started using the rock&plays instead. They sleep sooooo much better in them and it is awesome to reach over and rock them if they fuss in the middle of the night. Right now, it's the only place they sleep but we are going to start transitioning them into the crib very soon.
-A twin nursing pillow!!! This is probably equal to the rock&plays. I have a My Breast Friend twin pillow and I could not imagine nursing twins without it. It's not comfortable to lean back in so I have tried figuring out other set-ups when I'm really tired but it's really difficult to maneuver and place two (usually crying and squirmy) babies on a bunch of pillows. Like, REALLY difficult.
-The SnapNGo Double stroller frame. It's the one you put the car seats on. This thing is the only reason I can get out with the babies without help. It's super light, doesn't take up a ton of space, and is easy to fold/unfold/put car seats in. Car seats are heavy and it is nearly impossible to carry two, with babies in them, plus a diaper bag. The stroller is tandem and not very easy to maneuver, but it beats the hell out of lugging the seats and it has a huge basket (even though it's a bit hard to get things into and out of it). We did get a super-nice double side-by-side but the kids are still a bit too young to use it for more than walks in the neighborhood and that thing is HEAVY! I have a hard time getting it in and out of my SUV trunk by myself. At this point its so much easier to leave them in the car seats. Bonus: it's only $99 and you can sometimes find it used for really cheap.
-Muslin swaddle blankets. We love the Aden&Anais. They're super soft and thin enough to use in summer, but can be as warm as you need them, too. They're great as nursing covers, stroller/carseat covers, and as impromptu burp cloths. Best baby blanket to have.
-Activity mats. Get two different ones. They didn't have much interest in these at first, but it was a good place to lay them down and the music would keep them quiet for a little while... now they love laying on them and looking at the lights and toys and hitting/kicking the low ones.
-A changing station in the living room (or where ever you plan on spending most of your day). It's hard to carry two floppy babies and you're not going to leave one in a different room to change the other... not to mention they will usually both need to be changed. Those changing mats are okay, but I really prefer a designated, solid set-up. We're using a changing pad on the sofa table behind the couch.
-Boppies. I used them a lot in the beginning to prop the twins up for naps on the bed because they hated sleeping flat. When we started using the rock&plays, the Boppies went unused for a few weeks, but now that the babies are becoming more alert, we prop them up in the Boppies on the couch and they love to sit and look around or interact with us.
-Baby carriers/wraps. I didn't get one at first because you can't carry twins anyways, but you will need one... or two, or four. Harrison is an "in-arms" type of baby so there have been days that the only reason I can get something to eat or even go pee is because I put him in the carrier. I started with a Moby, which is great but very hot, then got a mei tie, a front-to-back type carrier, and an Ergo that Patrick uses. My favorite right now is the mei tie because it's easy to use and cooler than the Moby. The babies LOVE when Patrick puts them in the Ergo and wears them while he cooks and sings to them (he can only do one at a time so I'll have the other one). If all else fails, putting them in the carrier will usually calm them and put them to sleep.
That's pretty much it. As far as swings, bouncers, noise machines, etc.? My advise is to wait until your babies are here, then take them to the store and try out to see if they like it before you buy. Or just buy one and see how it goes. Olivia loves the swing but Harrison hates it. He ended up liking a bouncer but we tried 5 at the store before he liked one (it had to have the right amount of bounce and vibration and be at the right angle... he's picky).
Baby stuff is so expensive and there is no guarantee your baby will like the "latest and greatest". Also, don't be afraid to buy second hand. Most things are outgrown quickly or rarely used and you can get awesome deals.
I'm sure there will be more things as the twins grow. Anyone... feel free to add your baby essentials in the comments!
Thank You, God, for our precious miracles!