>Yes but you didn't ask the proper question, what do they do with the more intelligent twin? What do they do with the dumb twin?
These are my assumptions, but I would welcome you shining more light on the pair.
I expect that the more intelligent twin is raised to fill the role that this person was intended for. Intended by breeding and by any rituals surrounding conception time, etc. The dumb one, who may not be much dumber, would be detected by various competitions as the children are raised. The dumb one is the one who loses. In general I assume they would become more like servants to the house, but could play a role in breeding thus freeing the intelligent one from the need to do so. In other words, the intelligent one may be outwardly in a normal relationship, but does not actually breed with their partner, because their twin breeds with the partner's twin (if the partner is the opposite sex) or with a sibling of the partner (if the partner is the same sex).
I also expect that part of the service of the dumb one, is to roleplay the intelligent one in tedious public duties where one must be visible to the outside world. The intent being to free up time for the intelligent of the pair to devote more time to learning and self-improvement, meditation, planning, etc.
At the basis of my assumption is recognizing that all of us are actually playing a role, so it seems to me logical to use an identical twin to enhance someone's role so that more can be accomplished by that role in one lifetime than would otherwise be the case. In other words, there are two actors to play the role at different times, for public consumption.