When I researched options for a double stroller for my two (currently 4 and 2 years old), we chose the Phil & Ted's. Watching people in malls and stores maneuver their strollers, I definitely didn't want a tandem stroller as it was too long to get around. Nor did I want a side by side because they are too wide and simply don't fit around racks in stores. The Phil & Teds has a "jumpseat" that can be moved to accommodate two kids. When the baby is an infant, there are two options: I believe they sell a bar of some sort to hook the baby carrier onto. Option B - put the baby in the back of the stroller - either laying down or with the car seat inside. My Graco baby carrier fit right inside the back of the stroller with no attachments - easy to carry baby from place to place while still in carrier - it just slid right into position. As he got older, I would place him in there laying down. Then when he was older, we began using the jumpseat. I have been EXTREMELY SATISFIED with this stroller. We receive rave reviews from everyone everywhere we go on how unique it is, how smart the design is, and how maneuverable it is (there is one wheel in front which makes turning the stroller EASY). It's easy to use, doesn't require massive strength to maneuver (it's actually much easier to move than the Graco I have), smart looking, and has accessories (cup holder attachment and side saddle bags to store items in). Since the design accommodates two kids easily with no bulky stroller, I recommend those two accessories since with two kids there isn't as much "cargo space" as in traditional strollers. Bottom line - we LOVE this stroller. It's well worth the money - and I see these strollers more and more each day.
Here's a link for you:
http://www.philandteds.com/
This link has a moving frame that shows two kids in the stroller.
Everyone has their own price range and a feel for what they are looking for. I was more than happy to pay a bit more for something that was easy to move, easy to convert, and easy to take in & out of the car. Knowing I didn't want a tandem or side-by-side stroller, this one has been perfect for us.
Good Luck!