Tapping a humbucker means shorting out one of the coils to make it function as a single-coil pickup. The connection taps the pickup at the midpoint of what was originally two coils connected in series. 4-wire setups were a later development. 2-wire humbuckers are almost always internally connected in series.

Splitting means connecting one coil of the neck humbucker to one coil of the bridge humbucker (for example). You get two coils acting as a humbucker, made by splitting two pickups. Obviously, 2-wire humbuckers can't be connected this way.