I finally managed to do the double-slit method properly, as opposed to some crazy ad-hoc solution. ...And then I managed to punch through the bottom layer without realizing it, and had to improvise anyways (by which I mean designating the (full) stairs (or rather, the z level above) as water sources and telling my dwarves that the cliffside was an empty pond and they should go fill it, like, right now).
This fort is going weirdly, in general. I got my farm and still and kitchen up early (while I was waiting for the many steps of aquifer-punching), and didn't have much of any rock (I brought 10 mica. That's it - I spent 6 of it on two levers and links, and 1 on blocks for a temporary drawbridge (using one of said links), and 2 on cage traps, leaving 1 for a mood (or furnace) if necessary). I also actually built a surface structure for once (out of wood). I had to hunker down quickly, as there are two towers as neighbors. And I actually started an egg industry, for once. Although a) I brought 10 males and 2 females instead of vice-versa, and b) the females don't seem to want to use their nest boxes (I don't know why?).
Also, PSA: going into announcements.txt and changing building cancellation to pause-and-recenter really helps with aquifer-punching. Although it'll make you want to strangle your brewer and/or planter.