They probably could have stayed in their infant seats until after they are one year old (they are tiny!), but we decided that the new twins needed the seats more than we did.
I did a lot of research and decided to buy the Sunshine Kids Radian 80 car seats. They are slimmer than most car seats and fold flat when you need to store them. They were kind of expensive, but we needed to be able to fit 2 babies in the back seat of my car and allow space for an adult back there. Of course, the day after I bought them, Nana found the Eddie Bauer car seats at Sam's Club for $99 each. Those are so cool that they hold kids up to 100 pounds. We decided to put those in Daddy's car (that is a story for an whole other post).
Here is what the Twinsies look like in their new fancy car seats. They love sitting next to each other and they can play without having to reach over stuff.