So the next big decision (the only decision?) is what will your kids sit on? Chair or wagon? By wagon, of course I mean trailer. They range from cheap to expensive. In my case, I found the back pulling sensation very annoying. A few pedals stokes forward and a few tugs from the trailer behind.
Nice though that one parent can take both kids on ride. BUT, only on trails, or places with lots of room. I wouldn't take a trailer on some of the busier streets in our neighborhood. I borrowed my sis' trailer to see how it would work before spending $500 on a nice unit. Glad we borrowed first. No, the wagon trailer wouldn't work for me.
Besides, as a kid I remember rides in the chair. So we bought two CoPilot Limo model bike seats. They're fantastic. Kids love them, get a great a view, we can ride with them everywhere we want. All said, a much better option for us.