If Two-Face’s personalities had somehow been separated into two different bodies, how would Riddler react? Would he try to put them back together?
That would depend on the nature of the separation. Harvey and Harv are halves of a whole, each accounting for different facets of one personality. Both sides of Two-Face have tried to get rid of the other, only to discover that they are, in fact, one guy. They fit together and have always needed each other. With this in mind, there are two major ways they could be divided, which we can call an organic split and an absolute split.
The organic split assumes that each of the two different bodies has one permanently dominant personality and one subdued one that each add up to a complete person, able to function and take care of themselves. Something akin to adapting to the loss of one of the senses. In this scenario, Harvey would likely attempt to return to his old career and life before his accident, and Harv would run amok. They would part ways and keep their distance from each other, which would be a net loss for Ed. Both Harveys would still like him, but their hatred for one another would result in an awkward and unpleasant situation all around. Something something divorce leads children to places.
The alternative is an absolute split, in which Harv and Harvey just divide down the middle into two bodies. In this situation they would have to remain codependent, as neither has the adaptive tools to exist alone. They would lose access to each other's thoughts and the built in empathy of sharing a body, which would make their already uneasy relationship more of a struggle. While Ed might enjoy the idea of having two Harvey Dents to fawn over him, he wouldn't actually like watching them suffer and would look for a way to recombine them.
