No they do not just hit the first guy. Sometimes you want to open a specific hole, so you have to get leverage to one side or another. Also, the FB plays in as he makes have to seal off as well. Finally as you mention with pulling, you have to know where the other guy pulls to or it's pretty easy to run into each other. You have to set up "lanes" and stay out of the FB, RB, and QB space as well. It really does get quite complex and not as simple as knocking the other guy off. Also, what if the defense is stacking a LB over a shoulder of a Dline? Who blocks who? Again, a lot goes into it.