There are certain rules to follow when having two bass players in one band. The bass players have to know when to fill in the holes of the music as not to overlap. For an example, the bass players have to listen intently to match the volume of the other bass player so that the sound will not get drowned out. Also, when two bass players are in the band, the three octave ranges cannot be too high or too low. There should be an agreement between the two bass players on the form, harmony and rhythm of the song.
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John Goldsby’s Jazz Concepts – Two Basses Are Better Than One.