From what I've found, bullet penetration is based on the surface properties of the texture it penetrates. For props, I believe it's based on the prop_data. When a bullet enters into occupied space, it assumes everything is of the same material type until it reaches the air again.
So even if you sandwich concrete between two panels of wood, it will treat the entire distance as wood. Though if there's a break of at least 1-2 units, it will properly account for the material change. A bullet loses more damage when penetrating consecutive walls compared to a wall of equal width.
You can find all of the surface densities in csgo/scripts/surfaceproperties_cs.txt. Had you been able to define custom ones I would have suggested that, but there's no method of doing so.
In my honest opinion, the space sandwich should be treated as a last-ditch effort as it makes a mess of things. You really should just go about making the walls more thick or placing fitting props to block instead.
