There are a lot of different ways to do it, but the key is usually in the setup.
“Put it down!” Jason’s hand clenched around his pistol.
“No!” Jemima’s fingers twitched on the trigger, hers pointed directly at Kyle. “You first!”
“Guys!” Kyle yelled. His pistol remained trained on Jason. “Guys! Don’t do this!”
Or: they shoot each other.
The idea is to present it as a simultaneous action. Mexican standoffs like in Reservoir Dogs are immediate and surprising. You don’t have time for a wind up where you describe each character individually firing. They have to do it together. It’s not a series of individual actions, it’s a single action. This is especially important when dealing with firearms and explosives because it’s just so fast. A character who gets stabbed with a sword has time to realize that they’ve been stabbed. A character who gets shot, it’s going to feel more like “right here, right now”.
So, instead of trying to put the characters on separate lines, put them on the same line. In the same sentence. In a fight scene, the sentence conveys time and tells us which action happens first.
Character A shoots Character B. Character B squeezes the trigger, shooting Character A.
With two separate sentences, you feel the time delay. One action relates to another. One precipitates the other.
Character A locks on to Character B. Character B points his gun at Character A. Their fingers each squeeze a trigger. They fire.
Here, it happens concurrently.
The trick as I said in the beginning is in the setup. What makes Reservoir Dogs so successful is the way Tarantino builds the tension preceding the point where the characters shoot each other. The part where they shoot each other is the payoff and release of the tension. We go round and round, faster and faster, until finally, inevitably, bang. It explodes.
The trouble with taking Reservoir Dogs and looking only at how to write that specific moment is you’re missing the rest of the movie. What makes a scene like that work isn’t just the words you put on the page for a singular instance, it’s that the violence in that scenario is the payoff which your story has been building toward since the very first sentence.
Throughout the movie, we watch as the way out closes, as events go wrong, as the characters circle toward this point. Tighter and tighter, faster and faster, as paranoia spirals inward, where they don’t know who to trust, until the final scene becomes the breaking point where the work of multiple characters throughout the movie is put to the test. As the audience, we become similar to drivers on the road slowing down to watch the accident. But, you need to remember, the character drama is the reason why we can’t look away.
Violence is a means to an end. It’s a tool to release narrative tension. What makes it successful are the moments leading up to that explosion, the people involved, what they want, what they believe, and what they will do to achieve their goals.