You will want a seat that of course will keep your twins rear facing until past thier first b-day. Most seats on the market rear face to 33lbs, and a few rear face to 35lbs. ITs best to keep them rear facing as long as possible, and the AAP is even toying with the idea of upping thier suggestion of rear facing to a year, to rear facing 2 years. Test after test has shown the huge difference in accidents between children who were forward facing before age 2 and those who were rear facing longer.
The type of seat your are looking for is a convertible, meaning it will rear face AND forward face. I wouldn't bother with the 3-in-1 seats, that are a rear, front, and then booster. These usually do not make good booster seats because the frame of the seat holds the seatbelt away from the childs hips, and the seatbelt needs to be tight against the hips. For this reason I always recommend parents buy a dedicated booster seat when the time comes.
I would recommend any of the Britax seats because they are convertibles. There are a few higher weight harness seats on the market but they are currently only forward facing models.
The Britax Marathon is what we own. This seat rear faces to 33lbs, adn forward faces to 65lbs with a five point harness. Heavy straps and thier HUGS system make so the straps never twist. EAsy in and easy out. THe top slots on this are 17 inches, so your child could easily ride in this seat until he is 5 or 6. My oldest is almost 5 and he still easily fits into his Marathon, and he's very tall.
The Britax Boulevard is also a spectacular seat. THe only difference in the Marathon and the Boulevard is the Boulevard has true impact side protection, meaning it has those larger 'wings' that stand out by babys head to protect thier head from flying about in a side impact. Another way that Britax has one - upped the american market carseat.
www.albeebaby.com has Britax seats for $209 and free shipping, this is your best bet. STellar customer service, fast and free shipping. Most stores carry these seats, the Marathon and the Boulevard, for $270-$290, so getting this deal at albeebaby.com is excellent. And since your buying two you'll need to save some money!
You can buy any convertible seat, they are all tested and meet at least minimum safety standards. As long as it has a five point harness, its installed into your vehicle correctly, and your use it right every time, your child will be safe. The main reason I recommend the Britax Marathon or Boulevard is because they forward face harness to 65lbs, and its well known (or should be by now) that the longer a child is harnessed the better, that car seat belts are just not made for children, and children shouldn't use them. Britax has made great strides in keeping up with the demand for higher harnessing seats. While thier price tag is a little of a sticker shock, its better to just buy it now instead of getting two years down the road and facing your child outgrowing a different seat and needing a higher harness seat anyway.
Another thing to consider when buying a carseat is the top harness slot. Most seats are 16 inches, but quite a few of the popular ones (Graco) have a top harness height of only 14 inches...most kids outgrow this before they are 3! The Britax Marathona nd Boulevard have 17 inch top slots. Just antoher thing to think about if you go with another seat, measure that top slot and don't buy anything under 16 inches or your child will outgrow it in height long before weight and much too long before being to safely ride in a belted booster.
If you have anyquestions feel free to send me a message. I'd be glad to help in any way possible.