The rules for combat are pretty clear when it comes to applying wounds against models. Theoretically you are supposed to resolve each and every hit separately, so if you are firing ten bolter shots at a target unit you should roll one shot to hit. If it hits roll that one to wound, if it wounds the target player rolls his saving throw and if that's failed, apply the damage and go on to the next shot. In practice you are allowed to roll all the shots at once, roll all the to wounds at once, roll all the saves at once and apply all the damage at once. If the target unit's models have multiple wounds, the damage is applied to one model until it is killed and then damage is applied to another model until it is killed and so on. The defender cannot spread out damage over multiple models to keep them alive.

Heavier weapons which do multiple damage points need to be rolled separately, because excess damage from a single hit does NOT carry over from model to model in the target unit. If a lascannon hits, wounds and does 6 damage points to a unit of Ork boyz it does NOT kill 6 models. It kills one model really dead.

But I just thought of an odd situation involving Medics and Apothecaries which can heal wounds and possibly revive dead models. Suppose I had a unit of Space Marine Terminators who each have two wounds. Suppose the unit took three points of damage. One terminator model would be removed and there would be one more wound placed on another model. But then suppose I had an apothecary close by and it successfully revived the dead model. The rule states that the dead model is restored with a single wound left. But there is already another model in the unit with only one wound left. You aren't supposed to be able to do that. You can't spread out the wounds on multi-wound models. So what happens? Should there be a triage rule where the medic has to heal the single wound on the one model instead of trying to revive the dead trooper?