2v1 Defense
The first thing that the Cover player needs to do is to get into position. If he already is goalside of the ball, then it is a simple matter to shift up and over to establish the correct position. However, if he is upfield from the ball, then he must make an angled or looping run as he comes back to get into position. Why? He wants to keep his eye on the ball at all times, and he wants to be able to provide backup in the event that the Pressure player is beaten. As a result, he needs to estimate where along the path towards goal he would have to be to intersect the attacker if the attacker quickly got past the Pressure player, and then start heading for that spot. Then, as he sees that the Pressure player is managing to contain the attacker (so the risk of a break-through has gone down), he starts to angle around towards his ultimate Cover position.
Once in place, the Cover player becomes the "boss" of the defense - and it is his job to instruct the Pressure player on what to do next. Why? Because the Cover player can look around, while the Pressure player should have his eyes glued to the ball.
While positioning is important, the real work in training the Cover player involves training in communication skills. The Cover player provides the same type of support for the Pressure player that the 2nd attacker provides for the on- ball attacker. His job is to serve at the eyes/ears of the on-ball defender - and to provide clear communication about the best way to defend. Of course, his first obligation is to announce his arrival by shouting "Cover" when he is in place. Normally, his first instruction will be to start trying to steer the opponent towards one touchline ("Take him left" or "Take him right"). If possible, a right-footed attacker should be steered towards the right side of the defense (i.e., towards the attacker's left), in order to force the attacker to use his non-favored foot. Then, the Cover player starts to give instructions to the Pressure player on ball-winning ("No No" or "Now"). or "Not, go"). Normally, the Pressure player should wait for a signal from the Cover player before moving in to tackle the ball - so that he can be sure that the Cover player is ready to spring forward to provide instant Pressure if the tackle doesn't work.
Of course, the Cover player must be especially alert when the attacker is within scoring distance of the goal and will have a decent scoring chance if he manages to beat the Pressure player. This is especially true when the attacker is coming in centrally, so that the attacker has the full goal to shoot on. The Cover player knows that, once the attacker gets the ball within easy passing range of goal, it can be a simple matter to slip the ball between the legs of the defenders and into the net. As a result, if the attacker hasn't made a mistake which can be capitalized upon (such as putting the ball too far out in front) by the time that he has entered the PA, the Cover player will need to consider orchestrating a double-team rush on the attacker. When should this be done? Usually at the time when the attacker is approaching the penalty mark, and has just stepped on his shooting foot (so he only can get off a hurried shot/pass with his non-favored foot before the two defenders converge on him). The decisions involved at the same as in making the rush in individual defending. The only real difference is that the two defenders combine as they are going in, so that they can overwhelm and bottle up the attacker and/or put so much pressure upon him that his shot is easily saved by the keeper.
When the attacker is coming in from an acute angle to the side of the goal, then the defensive job is much easier (particularly until the attacker gets close enough to the goal to have a near post shot). The Pressure player will try to steer the attacker towards the endline, while the Cover player shuts off the funnel by positioning himself somewhere in front of the near goal post. This leaves the keeper free to take the middle of the goal (to be able to push high far post shots over the bar). In this situation, because the Pressure player generally can kick the ball out, the real interest of the Cover player is to keep the attacker from coming between him and the Pressure player - which will leave a big hole. Thus, he will want to close in a bit, and simply use patience to wait for a mistake.
When the attacker is coming in at an angle of 40-60 degrees, the defenders have a difficult choice to make. If they over-commit in trying to steer him to the endline, he may be able to spin over the top of them towards the central goal area and have an unobstructed shot on goal. On the other hand, if they over-protect the central area, they leave the wing area open for an attack. As a result, they usually will want to position themselves directly along the angle of his path. If they cannot close on the attacker before he clears the near post, then the Pressure player must drop down to cover the near post (so that the keeper can move more centrally) and the Cover player will need to slide in to block shots on the central or far post areas. This is a tricky time for the defenders, because failure to move together will create a momentary gap which can allow a shot (or hard near post pass) or allow the defender to slip through altogether.
In addition to training the players on how to set the proper angles in these situations, it is important to train them on recovery runs. When the Pressure player is beaten, the Cover player immediately steps up to provide Pressure. At the same moment, the Pressure player must INSTANTLY make a recovery run to get into a position to intercept the attacker if he beats the former Cover player. The most dangerous instance in 2v1 defending is during this transition time. If the former Cover player does not close quickly and carefully, both defenders will be beaten - unless the former Pressure player gets into an immediate backup position. Usually, the best option for the recovering defender is to make a sprint at top speed to an interception point on the line towards goal, while turning his head to see if further adjustments are in order. In general, the depth of the interception point should be set as deep as possible, while still permitting the defender to reach the attacker before he is likely to shoot. Once again, if the recovering defender sees that the new Pressuring defender has contained the attacker, he can flatten his run and come back to set up a Cover position. But, his first thought must be to get into a position where he could stop a shot (or intercept a cross, if a supporting attacker is in the picture).
As can be seen, even 2v1 defense is not particularly simple - and it can take a number of seasons for players to fully grasp how to handle the various options (and how to communicate well). However, smart players end up discovering the most of these principles on their own by being placed in these situations frequently. Hence, a smart coach will try to find time in practices for small group games or drills where the players can experiment with their own solutions - and learn from their mistakes.