A double lung shot will kill an animal quicker than a heart or heart / lung shot.

The reason is simple. The lungs have more blood flowing through them at one time than any other organ in the body. As the lungs get pierced the blood pressure drops to critical levels instantly which in turn the brain tells the heart to pump faster and faster to bring the blood pressure back up. Obviously this isn't going to happen with a hole through the lungs so the heart keeps beating faster and very quickly deprives the brain of oxygen.

When the heart is hit it can't pump any blood with a hole in it so it only drains. This draining takes longer to deprive the brain of oxygen thus allowing the animal to live longer and run farther.

How long the difference in death is I don't know but it is faster with a double lung hit. But, either one will result in a dead animal. If I got the heart instead of the lung I wouldn't criticize myself at all. Besides you look good when you can show fellow hunters a heart with a nice hole in it.