Hi T.~
I asked that same question a couple weeks ago. Here is a link to my question and responses, and if you read through them, another mom linked me to another post asking the same question... hope you get the info you need.
http://www.mamasource.com/request/532535078123798529
We ended up getting the BabyTrend Sit-n-Stand Plus, which includes a second seat option. It was something like $165 from strollerdepot.com with free shipping.
http://www.strollerdepot.com/items/baby-trend-sit-n-stand...
We haven't had the opportunity to use it yet, but it was easy to put together, folds up reasonably well for a double (no bigger than my Graco travel system stroller), and my 3 year old loved getting on and off himself (when we were trying it out up and down our sidewalk.)
From all the reviews I read and advice I got, it seems the concensus is that the side-by-sides are much harder to maneuver through smaller spaces. If you are looking for a nice side-by-side, I suggest the Peg Perego 60/40. Neat feature of having one seat larger than the other to accomodate the carseat, then eventually the larger child, if you are still using it by then.
Best of luck with your decision!