My twins are now 9, (4 boys in all) and I don't remember them sitting still for much of anything at 18 months. (unless they were quietly getting into something they weren't supposed to! lol) One thing that you have to remember, and if you haven't discovered yet... you will soon, is that twins are a whole other breed. It's hard to explain.. they're just different. My mother just couldn't understand why my twins got into so much trouble as toddlers, you couldn't turn your back for an instant... but it was because they were TWINS! LOL! (tearing their curtains from the walls, emptying drawers if they had a dresser in their room, climbing out of their cribs at 15 months, taking off their diapers during naptime and smearing poop on the walls, getting into the cupboard and dumping an entire box of cereal in 1.3 seconds flat...etc) It was constant. They were always thinking up something and always on the go... I don't remember them really sitting to read a story until they were a little older and understood more... like maybe 2 1/2. I had my first child 2 years before the twins were born, so it was a complete culture shock for me. My oldest was always EXTREMEMELY well behaved for a little one, NEVER doing something that he wasn't supposed to and always very calm and sweet. Don't get me wrong, my twins are awesome little people and they always have been... it's just different.
One thing that I'd like you to keep in mind as yours get older, is to be very careful about the 'ADHD' label. Mine were also premature, and immature for their ages. In kindergarten their teacher wanted me to have one in particular evaluated for ADHD or ADD. They were only 5 yrs old. I fought it for months, refusing... knowing that they are just little boys... what 5 yr old boy sits still all day. Plus, I knew my twins were very active. Eventually I gave in and took him to a child psychologist to have him evaluated, because I didn't want to ignore a problem if there truly was a problem. The child psychologist said that he did indeed have ADHD with other behavorial problems developing. He put him on Adderall. My child went from a happy, (sometimes naughty), but very active child... to a zombie kid who refused to eat and cried if anyone touched him at all or tried to play with him. It was horrible... I tried giving him time to adjust to the meds, because his teacher said he was doing so much better sitting and listening in kindergarten. I gave it 6 months, and finally took him off of the meds because he was not a normal child anymore. The next year he started 1st grade, and was top of his class. Both of my twins are straight A students, going into 4th grade now, and are at a 7th grade reading level. So I guess that even though the "professionals" diagnosed him with ADHD at 5 yrs old... he really did not have it at all.
I know this is a lot of babbling... I just want you to know that these are the same boys who couldn't sit for a story at 18 months old. It doesn't mean they are prone to have ADHD (mine were preemies too) or that they will hate to read later.
Best of luck to you with your little boys.... it's always an adventure! :o)