The precision is more for the turning/not turning than the actual value of the speed.
So the only "precision" is maybe the response time in making the decision. You can't have precision in the value of yes/no piece of data.
If you do in fact need precise speed then using more than one magnet will only improve the response time of the sampling not the precision of the readings, and in return it may introduce errors if the magnets aren't exactly placed.
So what's it to be, turning detection or exact speedo? And if so how exact, over what speed range, and what response time is needed?
Rob