I have a 3y/o and a 6m/o in Manhattan. I use slings for my youngest (and did for the 3y/o when he was my only) and my 3y/o walks. 18months is probably too young to really walk anywhere but I imagine lugging a lightweight stroller up and down subway stairs with the youngest in a sling would be much easier than a double stroller. :)
BTW, do you use www.hopstop.com ? It's like mapquest for public transporation!