I try not to, but if the bad guy is a basically unintelligent creature that is motivated by hunger, then unless you go out of your way to distract that monster it's going to eat your friend on it's next action. Some of the most intense combat scenes we've had involve protecting a downed foe that is going to be killed if someone doesn't do something.
For intelligent foes whose motive is some sinister plan, they only bother to double tap PCs if they have the time to do it. Quite often their main motivation is to escape the scene because they know they've been outnumbered and outgunned, and taking a round to try to finish one single threat while being increasingly encircled and threatened by the rest of the party isn't logical. So it does happen, but it's a much rarer thing. I don't have the NPC play that way unless nothing else makes logical sense.
Basically I rely on the internal logic of the story.