The way I see cogenitors is that they're a downtrodden minority in their own society, valued solely for their genetic contributions and for nothing more. I would think, given their extreme minority status, that all they do is facilitate the making o' babies. Day in, day out. Hence there's little time for crazy things like education. You're sexually mature? Off to the bedroom salt mines with ya!
Actually, it wouldn't shock me - here's a fan fic plot bunny - if the Vissians weren't maybe trying to replace them with some sort of chemical factory hormones or enzymes or whatnot. If cogenitors are replaced by a shot of, say, progesterone, then they can all die out and be remembered centuries later as a curiosity of Vissian evolution. This slave caste would become almost zoo specimens. It's their minority (and necessary for reproduction) status that keeps them alive at all. Otherwise, who would want a child to be born if the fetus were shown to be a cogenitor?
That also makes me wonder, since the cogenitors don't bear the children, what do families do when Mom gives birth to a cogenitor (as, say, child #4 out of 5)? Surely you can't raise them with the boys and girls! Why, they've got intergender cooties!
Vissian society would be quite the thing to study.