As far as I understand it, let's say you reanimated Itachi. Sasuke, for arguments sake, doesn't have an eternal mangekyou, so he needs to switch eyes with someone. If he switched with Itachi, Sasuke would have good eyes, and Itachi, the worse ones.
I'm almost certain you can't reanimate a person twice at the same time, otherwise we'd have seen a lot more Kages in the fourth shinobi war. This means because you can only revive someone once at a time, and upon death/revival the reanimated body's eyes flake off and diminish, only four of a certain set of eyes can exist at a time because the eyes destroy themselves when the reanimation is destroyed.
This means, at a time, you can only have 4 eyes in existence (Two in the reanimation, and two in the reanimated persons original body in a coffin, if they haven't rotted away already) .