Re: Stacked coils in series.
2 stacked singles with the top coils in series would work, but it would be no different than setting up a standard 3xS Strat so the bridge and middle were in series, assuming the middle was not RWRP.
Unless you're talking about putting 2 stacked singles in a humbucker slot. That can be done, and you'll end up with "2 singles in a humbucker slot" tone. If they're both high output, you'll get a lot of volume. If they're both low/medium output, you'll get medium/high volume, but either way retain the tonal aspects of having a single coil in the bridge (or whatever slot).
Assuming this is what you're after, you could put 2 mismatched stacked singles in the bridge slot, with the hotter one closer to the neck to get more bass/low mid punch.
A few years ago I did a similar mod to a 1990 Jackson Warrior, which had 2 slanted single coil routes at the bridge (like those Gibson Nighthawk pickups), where I put a HotRails Neck closest to the bridge and a JBJr in the next slot, and wired them in series (or at least both blacks to the same switch position). Gave that guitar a nice beefy punch-in-the-face tone that I've not gotten from another axe.
However, since those two were essentially miniaturized humbuckers, the coils that faced each other were of opposing polarity, so with stacked singles, you might want to look at the idea of one of them being RWRP.