My twins were born at 37 weeks. One was 6 lbs 11 oz, and one was 5 lbs 7 oz. I had a planned c-section, because I had a liver disease called cholestasis that was a danger to the babies. So I might have carried all the way to full-term, given the chance.
I did try very hard to eat tons of protein, like the PPs said, and to gain a significant amount of weight early in the pregnancy. I strove for 20 lbs by 20 weeks. I actually only gained 45 pounds total, and I beleive that's one of the reasons DD was so small. At the end, I was so big that eating any quantity was uncomfortable, and the heartburn was murder. I wish I'd gained more early on.
Mine had a slightly rocky start, but basically did fine. DS was born blue and had trouble breathing on his own, but he was only in NICU for less than a day before they let him stay with me. And DD escaped NICU by the skin of her teeth (and one ounce of weight-- she was 4 lbs 15 oz at her smallest, and they said she had to be 5 pounds to go home, but my midwife went to bat for us.) They both had trouble learning to nurse and had jaundice (DD's enough that it was alarming even to me) but were otherwise basically healthy and went home when I did. I think had we let them "cook" until they were ready to be born, they would have had no issues at all.
As far as beds, I found it impossible to get any sleep at all unless we coslept. I was nursing almost constantly during the night. The only way I could sleep is if I latched a baby on and slept while the baby nursed away, then roll to the other side and latch the other. If I had to get up and lift a baby out of a crib, I'd never have made it. I do use the cribs now that they're older; I have a full-size one where DD starts the night out (she ends up with us later) and a pack and play where DD naps during the day. I have not been able to persuade DS to sleep anywhere other than my bed. But yeah, if you plan on doing crib sleeping, they will probably be happier together at first, and then want separate beds later on.
As far as gear, it's better to get only one of each, and you can always get another of something if you find you can't live without it. I did use baby gear and strollers a lot more with twins than I ever did with my singleton, who I just carried or wore most of the time. The double stroller has been invaluable to us in getting both babies to nap at the same time, too.
I have never gotten the hang of wearing two at once, but I know some people do it very successfully.
And don't worry about the weight. I'm down to ten pounds BELOW the weight I started out at, between the demands of nursing two and chasing these kids around all day!