The original Fender P bass pickups use alnico V rod magnets, and no magnets on the bottom.
This is the bottom of a 1974 P bass pickup:
So the poles you see there are the magnets.
The cheaper Fender pickups use steel rods for poles with ceramic magnets on the bottom. Some other pickups like the DiMarzio Model P also use steel poles with ceramic magnets on the bottom.
With the pickup that use ceramic magnets, the ones with two magnets have the magnets facing the pole pieces with the same polarity, so that polarity is presented to the top of the poles. A cheaper version will have one magnet on the bottom. The two magnet version is usually stronger.
As far as tone, both ways can sound good depending on how you wind the pickup.