You need a Phil and Ted's with the extra seat on the back. It's the one and only stroller we bought and it has seen us through two kids, more miles than my math whiz of a 5 year old can count and we still love it. My husband and I are avid walkers and we'd throw the kids in the stroller for our long walks (although our 5 year old now prefers to ride alongside on his bike).
The Phil and Ted's has the footprint of a single stroller because the extra seat attaches to the back, and the child's feet go into the little basket under the front seat. I know you mentioned side by side strollers, which this is not, but we never had the issues so many friends had with the kids poking each other or one waking the other up.
It gets through every door. :D
Good luck choosing!