The purpose of 2 spindles on the Liahona

The purpose of two spindles on the Liahona was to allow Nephi and his family to know if the Liahona was working properly.  When pointing in the correct direction, the spindles would align.  When unaligned they would not know which direction to go.  As such this ball would also serve as an indicator of their level of obedience.

See:

http://maxwellinstitute.byu.edu/publications/jbms/?vol=3&num=2&id=59