My oldest was not a premie, he was 8 lb 1 oz at birth. He would never had fit in 24 month old clothing at a year, 2 years, yes, but not a year. I rarely buy more then a season ahead for first babies because you just don't know how they are going to grow.
My suggestion would be to give them a gift now, something small like an animal for their cribs if allowed or something more practical like gift certificates to the hospital cafeteria, gas cards, etc. They will be driving back and forth to the hospital a lot and probably eating more meals then they like in the cafeteria. Then, when the babies come home, you can get them something fun like clothing.
If you really want to do clothing now, get some non-onesie t-shirts, some pj type outfits that snap up the front, no zippers or back closure. There is a really good chance the babies will come home on Apnea monitors and dealing with the wires, etc. can be a pain with some clothing styles. I would get those in 0-3 month or 3-6 month sizes.