For one remember. At the core you do attacks one at a time. Rolling multiple is for sake of CONVENIENCE. Nobody wants to go through 50 dice roll to hit, wound, save, damage, next
So you choose order with which you attack. Against individual target you shoot all weapons you allocated vs specific unit first. Then the other unit. Then inside unit following quote(page 20) is relevant.
If your unit is shooting more than one ranged weapon at a target, and those weapons have different profiles, then after you have resolved attacks with one of those weapons you must, if any other weapons with the same profile are also being shot at that unit, resolve those attacks before resolving any other attacks against the target.
So basically you roll all shots of X type and do them and then move to next weapon.
So as attacker you can choose order. Then they are done completely. Then move to next weapon.
So opponent can't wait because first weapon has to be resolved completely.
However attacker also can't do "first bright lance, then finish with shurikens, then next bright lance". He also has to decide order before rolling all so if you are rolling multiple weapons together(with different coloured dice representing different weapon) remember you have to declare order FIRST. Can't wait to see result before deciding order you apply weapons.