Source: Warhammer Fantasy: 4th Edition

Dividing Shots
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A unit of troops always takes aim and shoots at a single target where possible, obeying the commands of their leader who is directing their fire. Therefore, where possible, all the unit must fire at a single target, such as a unit of enemy troops or a huge monster or chariot. This may occasionally mean you are obliged to fire at one target rather than another, because one target can be shot at by all your troops while another may be in sight of only a few. However, it is sometimes impossible for all your models to shoot at a single target because no one target is visible to all shooters. In this case you may divide your shooting between two enemy targets if you want to.