The second defender’s first job is to make the first defender braver, not steal their tackle.
If the first player is slowing the attacker down, the covering player needs an angle and distance that protects what happens if that first defender is beaten. Standing directly behind gives very little help; diving in from the side can remove both defenders.
I’d use 2v2 and 3v2 games towards goal. Ask the second defender, “If your teammate gets beaten now, are you in the game?”
You can also give the attackers a point for splitting the two defenders with a pass. That helps the defenders notice the gap between them.
At U12, pressure and cover are enough language. They do not need a long lesson on defensive-unit theory.
Coach the picture repeatedly and let the players feel what a useful covering distance looks like.
