Okay, what I found essential was......
a good pillow for tandem nursing (I used the EZ2Nurse pillow as mentioned by many.) I recommend having two covers (even if you layer burp cloths/blankets on it as recommended) and using the belt. I thought the belt was useless because it slipped all the time and didn't hold tight, but I think that was because I somehow "stripped" it or something early on. Postpartum, I still had belly sticking out there for awhile when I was upright (it would fall flat and wrinkly if I were lying back) and I guess moving or shifting with the babies on the pillow pulled at the buckle lock enough times and left it slippy so it just wouldn't stay tight. Then I switched covers after some sort of devastating poop/spitty thing and wonder of wonders, the second cover worked RIGHT and suddenly the pillow worked so much better for me! It was nice and snug and stayed that way! Anyway, that was my experience. Eventually the same thing happened to the belt/buckle on the second cover, because I would shimmy a bit once they were asleep and try to hoist them from the pillow onto Boppies on the bed, hoping for a successful transfer. Once they were in the 15 to 17 pound range, this was a lost cause (the baby still on the double pillow would wake from all the lunging on my part, and the baby I was moving would wake because there was nothing subtle about the maneuver) and once I felt the buckle slip during this and it hasn't held since then.
(Eventually I went with two Boppies on each side of me, with the twins in double football hold, when I was completely trapped in the EZ2Nurse pillow with no way out because the babies were too heavy to maneuver simply.)
4 Miracle Blankets. More would have been nice, even. (Just because of the laundry issue and newborn poo.....mine were swaddled a lot and odds were that somebody would get poo on their clothes and blanket. Nice to have a little more time before you HAD to do laundry to get the blankets back.) These blankets worked so well for my babies, and it is nice to have a quick & fool-proof swaddle when you have two babies eager to nurse (i.e., screaming) and you are trying to wrap them up.
I actually bought a white noise machine and I do still use it (it sounds like a tropical rainforest where we sleep.) This is helpful if you have older children around the home, and it just really helped to be more forgiving if one baby woke before the other....I had a little bit of time to get to him and whisk him away without his twin waking, if the white noise was going.
We bought a swing this time....and ended up getting the second swing (that I thought we wouldn't need.) This was great when I needed them to nap in different spaces (one baby would fall asleep but the other wouldn't, so I'd take the awake twin downstairs and he'd eventually go into the swing and sleep there, and I didn't risk waking baby A by trying to get baby B back in the same room after he was asleep. And we had one swing upstairs in the bedroom...there was a period early on when one or the other slept in the swing at night and sometime for naps. This let me nurse them individually (lying down) at night which was a lifesaver. So for me, the two swings were must-haves. I also used the downstairs (kitchen) swing in the evening, either during dinner or while I was cooking. Depending on the situation. We usually had one baby at a time who was happy to hang out there and watch everything, and my husband could hold the other one. The swing worked well when bouncies didn't always.
We had an infant-to-toddler rocker (Fisher-Price, like a bouncy seat but can be used longer) with my older daughter, and we got a pair to use with the twins. I like that even though they are not using it now since they are mobile, they will again (when they start walking and can get in and out of chairs safely), which is what my daughter did. She still uses hers as a rocking chair/seat (she's 5) which is why we didn't co-opt it for the babies and just get one seat. One is still in the box, though, because I found I didn't need both seats. (But they can both have rockers later, if they want.)
I really like our stroller but didn't use it until later, and it's probably not a must-have (the high end aspect of it, I mean. Nice, and now that I have it it feels essential, but I'd probably deal with whatever configuration I had.)
I did use a double snap 'n go stroller when they were in two buckets, which I only really realized would be helpful once I tried to get out on my own with them. There are other ways to deal, though, so I wouldn't call it essential. It was a lifesaver for me, but that was partly my situation.