After reading the DFWiki page on wells, and it's associated talk page, I felt rather confused and mildly curious. So I decided to test everything I could think of.

I made a super-deep well tower, 30 levels tall. If you drain the bottom out to one level, the very top well will still be functional, even though there's only one level of water, 30 levels below.

In that same well shaft, I built two other wells, directly below each other and the first one. All three well openings, even though they go through each other, are still functional.

Below that, again in the same well, I have made a hatch cover. If it's closed, it obviously obstructs the above wells. But if the lever is pulled, the hatch is held open and- HUZZAH! They function!

I made a couple of bucket-filled wells. I was going to experiment on how far away from a water source you can build these before evaporation ruins them. But, after making two, and watching the rate at which my dwarves filled them, I got the impression that the required distance to make a bucket-filled well unfillable, is pointlessly long.

Below the housing district, which is barely even touched as far as housing goes, is one, single, enormous reservoir, that is only one level deep. Four wells tap into this reservoir at the same time, from each of the four corners of the housing district. Wells are fine with drawing from a common water source.

I was in the process of making a perpetual motion waterfall that dumped water into the well, drew it back up, ran it under a water wheel, then dumped back into the reservoir. It doesn't work, though. I'm not as good of a dwarven engineer as I'd like to be.