A double layer of chunks is better then one to protect my paws during the shooting? The same is for a double layer of sandbag?
No you only get cover for the chunk you are directly behind. Other things can block line of sight, walls and such.
What it would do is give your opponent the opportunity to get cover from the double layer if they manage to get there.
What you gain though is a bigger obstacle for melee units to cross or circumvent to get you your units, which could net you another salvo before retreating.
As far as I know, multiple layers don't help.
Different cover has different defense though. Walls (you can keep corners) are great. Sandbags are also good.
Angles matter also - cover is only 100% effective if it is directly between you an the enemy, so very good cover is often alternating wall with sandbags (or doors - since it is very useful to be able to duck to safety if you are injured).
-Stigma
My favorite defense position is a three tiles wide wall with a layer of 2-3 chunks in front and on the sides.
The enhanced defense position is creating a U-Form by putting two walls on the side and a stone door in the middle.
I know that sandbags are better than chunks, but cost ressources while chunks are plenty and free.
Something else to keep in mind is pawns now climb up ontop the 1st chunk they find and continue at normal pace over subsequent ones, for true obstacles (to slow them down) you have to stagger them now to make them "climb down".