I hate to think about having to face that situation in real life.

On the lighter side, when I go bowhunting with any ofmy buddies (and this arrangement is just between us, not just any Joe Schmo)it's agreed that if one of us hits a deer, and the other guy has still has a shot at the same animal, he takes it down in an effort to recover the deer easier and sooner, but it still belongs to the first guy that drew blood regardless of the point of impactof either arrow.
If I was gun hunting with a buddy, we wouldn't be close enough to know if a deer belonged to the other guy or not, so this rule wouldn't apply, and we wouldn't want the additional loss of meat from a second shot unless the first hit was extremely poor anyway.

Thinking about the original question, I'd like to think that the 'killing shot' idea would decide the issue rather simply, but since all wounds to the major organs will kill, albeit at varying speeds, a 'killing shot' would be any other than a mere flesh wound to muscle, bone, and skin. I would guess that the vast majority of disputes would involve both hunters making 'killingshots' on the same deer, and the arguement woul dbe more about who made thekilling shot that would result in the quickest death.

Even if both hunters wanted to be fair, therewould stillbe grey areas determining who hit it where,whether the first hunter's shot would have killed it, and whether the first hunter would have been able to recover the animal with proper waiting period, tracking skill, and without interferance from the second hunter.

All good reasons to hunt as close to alone as possible unless you're hunting as part of a team or with buddy with whom you have an understanding.