... the more I study it, the more that actually makes sense.
If you only had 1, then you could have the target in the center of your cross hairs no matter where the gun was actually pointed.
If you have 2 that are adjusted to the weapon, then when you only see one cross, AND your target is in the center of that cross, your bullet should land nearby (still have to account for wind and drop, and every thing else)
Real life rifle iron sights usually have "2 sights". For peep sights, you have the rear sight aperture (a disk with a peep hole) and a front sight post. To aim, you need to align the weapon and achieve perfect sight picture (with the front sight post's tip at the center of the rear sight aperture).
So, looking at that orks sight. It's not as good a real life peep sight, because the sight itself blocks too much of your view. But as primitive as it is, it probably works.
The one in the way back though? That's for shooting at long range. Bullets drop the further away they travel, so the ladder sight is used to compensate for the drop (if you can judge how far the target is). essentially, the ork is only looking through 1 of the 2 rear sights at a time, and simply ignores the one he's not using.
Many real life machine gudn will have both a peep sight, and a leaf sight. The difference is that real life one's tend to be designed so that one sight can be flipped out of the way while the other is in use.
pic related is one such ladder sight / peep sight combination. (not pictured is the front sight post)
It actually drives me a little crazy when playing Borderlands (any of them), when there's a weapon that ONLY has a front sight post, or it has a front sight post and peep sight, but the two are never aligned! Every thing I know tells me it's wrong... but the rules of the video game are different.