Very cool shot!
Just a note... if done properly with a capable camera, timing isn't an issue with a night photo like this.
Set the camera up on a tripod, open the shutter and let them fire. Even if they don't fire at the same time, as long as the shutter is open, both shots will be recorded on the film and it will look like they fired instantaneously. Whoever fired first could even move out of the frame before the second guy fires and the photo would still be just like this one.
Not trying to denigrate the shot or anything... just providing some photography info to let you know that they didn't necessarily both fire at exactly the same time.