Stereo pickups can be used to split the signal from bass and treble strings of a guitar to different amplifiers. This means that different effects and EQ can be applied to the different string fields.
Pole pieces can also be removed from a 4 conductor humbucker, which can then easily be made to retrofit a LP style guitar.
Guitars with single coil pickups may have limited pickup combinations with this setup.
for example, A stratocaster:
Low Strings: Middle
High Strings: Bridge / Bridge + Neck / Neck
Humbucking pickups can be split, and used as two single coils. This could then lead to several stereo pickup combinations:
for example, a les paul style guitar:
Low Strings: Bridge / Bridge + Neck / Neck
High Strings: Bridge / Bridge + Neck / Neck
This style of modification is used by Jesse Hall of 'Experimental Dental School' to great effect. Jess uses a variety of string gauges, and breaks the pickups into thirds, so allowing him to play bass, rhythm and lead all at once on the same instrument through three amplifiers.
I have also designed and made a stereo guitar with rotated pickups, similar to the designs of Yuri Landman.
I should note that removal of pole pieces is much simpler to retrofit, as with the rotation method it is very hard to align the pole pieces beneath the strings.