Teamwork, at its basic, 2 men level. In this example, with an Archer. Hypothetical set up in this image:
A is an enemy group advancing. They are in an Open formation, skirmish line. B is your defender's. The defender's may be behind a barrier, may be behind a shield wall, could be behind a single tree that obscures the archer somewhat, or may be in the open.
C is your first designated target for the tandem. He may be the main threat, but because of his position on the open flank he is the natural target.
E is the Skirmisher who is working in tandem with the archer. He and the archer confer on who the target is and the tactic to be used. Timing is critical for any tandem operation. This is a sniping drill. F is your Archer. Depending on his skill level the Archer will tell his tandem mate how far out to perform the 'bait' action. Timing is critical to kill the target before the teammate skirmisher can get hit.
The enemy advances, the Skirmisher E moves out in a right hook to a position within the range specified by the archer, and comes upon the Target (C) from the right. The archer has his weapon notched and ready to fire but is facing to the left or is obscured behind the barrier/terrain, etc. He is; however, watching through perifphereal vision at his tandem mate, and has already 'seen' his next action through his minds eye. This way the action to follow will be more instinctive and quicker.
When E moves within range of the Target C he must be a convincing threat. He must move to within attacking range, but mayhap just ever so slightly out of range of his opponant. Threaten and make C prepare for engagement. C will move his shield and body towards E, thus exposing the entire oblique flank of his body to the Archer (F) who can now move to the right and fire a shot before Target C throws a hit to the skirmisher.
Should this result in a kill. Archer should reload quickly while the Skirmisher (E) works down the line from right to left of the enemy.
This tandem operation could be done by a shieldman with a tandem spear or Red as well. The action will be much closer to the line/barrier, and to eachother. Timing will have to be much better, distance must be paid attention to in great detail and you must be quick when you act. If the 'sniper' spearman or Red is too slow, the skirmisher may get hit and killed and the target will then change his shield and body orientation towards him.
Konradr