it's not that straightforward,the 2 CB's cannot simply stand in the centre of goal waiting for crosses to come in,mthey whole back 4 needs to be a unit,if one goes right,they all shift across.
look at it this way,if you have 2 wide attacking midfielders it is the same principle. when Pepe is running down the wing looking to make a cross into the box,one forward will go front post,the other forward will come central and Krasic would tuck in a little towards the edge of the box in case a loose ball misses the forwards.