I stopped a couple pushing one of these on the street last night. (It's a weblog-induced habit that no doubt embarasses some people in my family.)
Almost before I could ask what it was, the mother said, "It's not from here. It's French." As if they've had to answer this question a lot on the streets of New York. "Bebe Confort?" I asked, and they smiled with relief, as their explaining job just got easier. "Yes."
It's called the Twin Club, and they really loved it, and it apparently works great for twins in the city. Fairly light and easy to work with; I didn't ask about the subway, though. The Twin Club holds two types of Bebe Confort car seats; they had the smaller, more manageable Elios, not the giant egg-shaped Creatis. You can also snap on regular stroller seats (the front one seems to dangle in space) or a seat/bassinet.