**The** word for the military action of shooting several targets at once

Solution 1:

In this video, the words simultaneous and synchronized shots are used. However, I believe the correct term is Simultaneous Engagement, as seen here quoted from the book Advanced Sniper Markmanship, which I believe was written by an instructor in this field.

Simultaneous Engagements

In certain situations, it's useful for several snipers to simultaneously engage the same target or multiple targets...

Solution 2:

Perhaps it was time on target, defined by Merriam Webster as

A concentration of artillery fire on a target in which the time of firing by each unit participating is so regulated that all the projectiles reach the target simultaneously

Solution 3:

A sniper's shot that kills more than one target is called a Quigley.

The term comes from the 1990 film Quigley Down Under, in which the eponymous character dispatches multiple opponents with a single shot

This newspaper article, and this one refers to the term.