Getting around the city by myself with two kids is a bit tricky. We currently have three different strollers, each providing a different configuration and need for our daily commutes around New York City. You can totally survive with just one stroller and two kids but it is always nice to have the different options.
When both kids need to be sitting: Mostly for longer walking distances, we have been using the Britax B-Ready stroller with the B-Ready second seat on the bottom and the Britax B-Safe carseat on top. The Britax B-Ready is a modular stroller with over 14 different configurations, so when Baby Girl is able to sit upright (around 6 months old) we can re-adjust the stroller with the main seat on top and the second seat on bottom.
One kid is sitting and one kid is standing: Mostly for shorter walking distances, or when H walks some and rides some, when he is tired of walking. A few days a week H goes to a drop-off preschool program and I still need a stroller to use with the baby when he is at his school. On these days I have been using the Orbit Baby G2 stroller with the Sidekick stroller board attachment. The Orbit stroller is easy to push with the Sidekick board on and can still easily go up and down small curbs in the city. Plus when he is not ridding along with us I can just hook the Sidekick board up out of the way.
One bigger kid sitting and a baby carrier: For longer adventures especially when using the subway or taxi, I travel with the Quinny Yezz light weight umbrella stroller for H to sit in and the Britax baby carrier for LilStrange. This is also the easiest combination when my husband is around so one of us is pushing the Yezz and one of us is carrying the baby (perfect for our Sunday brunch and museum outings).