Here's something I found that is quite specific concerning the 2-man route vs cover 3.
"The corners must maintain outside leverage and use the free safety to their advantage. If they get beat to their outside, they have no help to rely on. That is why they use zone technique (back to the sideline) and widen with any vertical release to protect the sideline.
The free safety is the top of the defense. He must honor his responsibilities and not chase any intermediate routes. Offenses will run a deep dig (15-yard square in) to entice the free safety to jump the route — while running a post from the opposite side of the field, leaving the corner naked and playing from outside in with zero help. A classic Cover 3-beater."
Any kind of crossing route works the same as the dig route.
As I mentioned before, the only way the deep safety can step down on the intermediate route is to have the off-side CB to take over the post.
I will show a few more 2-man route.
On an 86-yd TD by Jennings (Packers) against the Dolphins earlir this yr, you will see that the on-side CB V. Davis plays exactly the same way Aso, Hayden, Mathis, or Jackson played.
They all have to protect the side line.