User blog comment:Turtle Cake Something/Why it May be Impossible for Two People to Kill Each Other/@comment-4295646-20130603194603

Real bullets aren't hitscan, you see. They have a travel time, during which it's perfectly possible to finish pulling a trigger. Probably happened all the time in duelin' days!

There's probably also going to be slight time discrepancies no matter what you do, since different physiologies might take different amounts of time to shut down entirely.