Buryit and mpsjr are correct: if you die while fighting one of the double bosses within a Realm of Evil, you are able to run back and take another crack at them, with the caveat that you should make sure your five stacks of Nephalem Valor are still active. The stacks need to last you all through the fight, because you'll need them whenever the final boss dies in order to have a chance at their coveted organ drop.

If you think the fight may run a little close, it's best to go and refresh your stack before starting the encounter.