Even the double-slit method can and will flop on its ass if you don't carefully micromanage the order of wall building.
You wanted a wall there? Sorry, that's a diagonal, you can't do that. You deconstructed the wall that was in your way? Now there's too much water coming and your pump operator can't keep up.
U MAD?
I've developed a variation on twin-slit which is far less micro-management intensive, you can do it blindfolded with a mild case of metal retardation. It's optimized for speed and sheer laziness, and there is basically two aspects which make it much faster and lazier:
1) QM's second method (for layers with aquifer below) is optimised to require no de-construction at all and far less construction.
2) To deal with the bottom layer, a cavern drain is installed as priority, since it's much much easier to work with aquifers with a drain below.
So far the only documentation is my video on youtube:
http://www.youtube.com/watch?v=TXQH9dliFZE
In the video, it only takes a little more than 5 minutes to get through the 2-layer aquifer. That's like, uh, 1/3rd of a season. I haven't timed QM's original method, but I know it's not nearly that fast.
It's possible to go through 1 layer aquifers in a couple of minutes on most maps. On cold maps, less than a minute. It's really all about knowing the techniques, then aquifers are a total pushover.
Did you rewrite the double-slit article on the wiki, or was it someone else? It's definitely changed since the last time I read it. Which was when I started a fort just to test doing it, the job cancellations were crazy. I think it was a 2-level aquifer, I was using the system the double-slit method now recommends for the last level only, don't know why I wasn't draining to lower levels at all... must have been because the wiki either didn't recommend that back then (this was May or June this year), or I didn't understand that bit.
Anyway, the article now mentions the option of draining once you get a 1x1 safe area at the bottom. However, it (and you) only mention draining to caverns). Wouldn't it be easier and safer just to dig a few z down, then to the map edge, carve fortifications, and drain through those? Or is there some sort of limit to off-map outflow, and the caverns are necessary because they usually have plenty of areas open to the map edge (as opposed to a 1-wide corridor to the map edge)?