Hellobee Boards

Our apartment when X was born was upstairs. We bought the Uppababy G-lite because it's so light and easy to carry up and downstairs. There's a strap so you can carry it over your shoulder up and down stairs, and you could hang it on a hook using the strap too. It can stand upright when folded, so it was really easy to store, if you don't want to hang it on a hook--- so shelving isn't necessary. The downside is that since it's an umbrella style, we couldn't use it until X was 6 months. That wasn't a problem though since we wore X in a baby carrier until then anyway.

We are moving to an apt on the second floor, but with two flights of stairs. We have a spot in the garage where we can leave the stroller though which was key for me (I was preg when we started looking so it was ver much on my mind).
I've heard the city mini is best for that since it is so light and the fold and unfold is SO easy- I practiced it in the store. I think as long as you can fold it and hang it or slide it next to the other one you'd be fine.