> Has anyone even built a two-story DC?

Downtown Los Angeles: The One Wilshire building, which is the worlds most connected building. There are over twenty floors of data centers. I used Corporate Colo which was a block or two away. That building had at least 10 floors of Data Centers.

I think Downtown Seattle has a bunch too (including near Amazon campus). I just looked up one random one and they have about half the total reported building square footage of a 10-story building used for a datacenter: https://www.datacenters.com/equinix-se3-seattle