I have a Burley Bee that seats two. We bought it when we just had our daughter, that was before my wife and I found out we were having twins! They're getting to the point where they'll be able to ride in it, I'm looking forward to getting back out riding.
I think a trailer would be best, unless you want to do an Xtracycle addition to your bike:
http://www.xtracycle.com/cargo-bicyc...eeradical.html
You'd have to buy the kit and the two seats. When you consider that even just getting two seats on their own would not be that cheap of a way to go, a brand new trailer would be a comparable option.