Things like different gauge wire, number of turns, insulation material, or magnet type between the two? Some of these might mean the end result isn't perfectly hum-cancelling, but it'd be interesting to tailor the bass/mid/treble response for each of the two coils.
I recently rebuilt a Z-coil the "other" way around (treble coil closer to the neck, bass coil closer to the bridge) and like the result, so I'm interested in variations. Specifically, I'm still looking to "tighten" the bass side a bit. Next step in the experiment will probably be to lower the bass side a but more than spec and raise the pole pieces so they're back to spec, and see if that does the trick. If so it'd make the first paragraph redundant - and now I'm starting to think out aloud as I type and ramble a bit
