In one of my favorite movie firefights - in Tears of the Sun - Bruce Willis and his small team of Navy Seals are caught under heavy fire by an overwhelming opposition. They survive (well, some of them, anyway), by tactfully holding the line and then retreating. Watching this scene made me think of a combat skill that allows you to coordinate your group in battle, called "Battle Tactics".
Since combat time is based on action/reaction, a leader or other combat unit can call out a command or request (e.g. "cover me") and the recipient(s) of the command will be able to react to it, rather than have to wait until their normal turn. Naturally, the team will have to be trained in battle tactics, otherwise they will not be able to coordinate; an unskilled combatant might think, "What? What does he want me to do?".
Commands vary in complexity, especially commands that require the coordination of several units at once, such as formations. When used properly, this skill will give soldier-type characters the edge in combat.
Example: Kanu Gon and Lo Din are behind cover and under heavy fire. When Kanu Gon yells, "Cover me!", Lo Din lays down suppressive fire, which forces their opponents behind cover, while Kanu Gon simultaneously runs toward a more advantageous position. When an opponent gets out of cover to fire back, Kanu Gon already has him in his aim and takes him out with a 3-round burst.
(Notes)
To elaborate on the mechanical side, yelling out a command will allow recipients of the command to react on your turn, or at the beginning of the next round. I'm still deciding whether to impose an action cost to it (probably).
Naturally, concerted actions will give you a significantly greater advantage than the sum of its parts. A gladiator might be able to block arrows being shot at him one at a time, but he is much more likely to get hit if all those arrows were fired simultaneously.
For allowing me to emulate concerted action so easily and simplistically, I feel vindicated about my combat time architecture.