2 ways, that I would shoose:

1. Trying to combine other animations: legs running, head talking, arm aiming...
2. Avoiding the problem: bone animations are 'cheap', add an animation with both arms aiming

Do you want to aim at different enemies at once?, like in Tomb Raider?

EDIT: Vergiß meine Vorschläge! Storm_Mortis Analyse ist einfach durchschlagend!