Aye, fortifications make it so a creature needs a high degree of skill to shoot through it unless they are directly adjacent to it. So doubling-up fortifications would make it near-impossible to shoot through unless the shooter is highly skilled, but this applies both to your own dwarfs as well as to invaders.
However, a creature directly adjacent to a fortification does not need to be skilled to shoot through it. This means that relatively unskilled crossbow dwarfs can position themselves behind a fortification and shoot with confidence that, unless their enemies are expert marksmen, they are relatively safe. But that means that invaders who can close the distance to the fortification can shoot back through it at them, since it works both ways. If the enemy is really skilled (like the leader of an archery squad) but your own dwarfs are not yet well enough trained, then they would be the ones at a disadvantage in a fire fight.
Best advice is to build your fortifications so you can only stand on one side of them to shoot. This could be something like a shallow moat on the outside edge of them, or it could be a raised area like a tower or wall that you build fortifications on the top edge of.
Incidentally, if you ever build exterior windows in your fort, I recommend you put fortifications on the outside of them. It prevents something from simply walking through and smashing them, and the windows will allow visibility without allowing projectiles to shoot through.