Who Would Die For Whom

A common situation in storytelling is for one character to give up his or her life to save another.  When done right, it's a powerful scene - the final act of a selfless hero.  The only issue I have with it is how common it is.

There are a few varieties of the heroic sacrifice, all of which are pretty common.  There's the non-death version, where the hero was committed, but ends up not dying.  There is also the refusal, where the person being saved does not want to be saved at the cost of the hero's life.  And there is also the reversal, where the hero ends up being saved by the person who was going to be killed anyway.

It seems to me that people who would do a heroic sacrifice are so close that either one would do it for the other.  This leads me to the question: What if two such people were put into a situation where one of them was going to die, but either one could do a reversal?  For example, if Bob was going to die, Alice could take Bob's place and die instead, but before it happened, Bob took Alice's place.  This cycle could go on indefinitely.

This scenario doesn't seem particularly compelling by its own right.  The ticking clock disappears and the emotional draw of a noble, impulsive decision disappears.  But it does have two qualities going for it.  The first is that it creates a different situation, where people have to rationally talk about death and actually measure the value of each other's lives, which does create an emotional draw all its own.  The second quality is that it is hardly ever done, which means it hasn't had a chance to become stale.

Try to make one of these scenes.  Create two characters (or more) who care for each other enough to lay down their own lives.  Then force them to decide who will die for whom.