I bought a guitar that had a Quad Rail in it. 24K full, 12K split. Full, it was very dark, warm, fat but not quite dull or muddy. Split, it was still pretty high output but sounded kind of thin and bright.
It was kinda crappy, but also kind of goofy over-the-top fun. ----------------------- It did make me think that having two different Strat-sized humbuckers side by side might be useful. Like say a Little'59 and a JBJr, for two different sounds from one pickup position.
not quite a double humbucker, but the Bareknuckle Black Hawk bridge is IMHO quite outstanding. triple ceramic magnets and 42 awg Formvar wire wound to 8.6k.