The rules for making attacks have been written assuming you will resolve them one at a time. However, it is possible to speed up your battles by rolling the dice for similar attacks together. In order to make several attacks at once, all of the attacks must have the same Ballistic Skill (if it’s a ranged attack) or the same Weapon Skill (if it’s a melee attack). They must also have the same Strength and Armour Penetration characteristics, they must inflict the same Damage, they must be affected by the same abilities, and they must be directed at the same unit. If this is the case, make all of the Hit rolls
at the same time, then all of the Wound rolls. Your opponent can then allocate the attacks one at a time, making the saving throws and suffering damage each time as appropriate.
Note that if all the models in the target unit would require the same saving throw against the attacks, and the order in which the attacks are allocated would make no difference, then your opponent can make all their saving throws at the same time, and can do so as soon as the Wound rolls have been made.