If I can ask a question, I just read the Mistborn trilogy and, were Preservation and Ruin two different shards or a single one with their power split somehow? If they were two shards, does that mean a single person can hold more than one, since Harmony apparently holds both now?
They were two shards.
Yes, one entity can hold more than one. Remember that holding a shard changes you, over time. Rayse knows this, and prefers to leave behind destroyed rivals as opposed to taking their power and potentially being overwhelmed by it.
I have a question, if you are willing. Would Ruin be more compatible with Rayse, would he pick up that shard had he visited Scadrial and shattered him? All the shards we have seen that he has shattered seem rather different in intent than him- Honor, Cultivation, Love, Dominion. But Ruin seems more in line with Odium. Rayse has ruined the days of quite a few people.
Technically, Ruin would be most compatible with Cultivation. Ruin's 'theme' so to speak is that all things must age and pass. An embodiment of entropy. That power, separated from the whole and being held by a person who did not have the willpower to resist its transformation of him, led to something very dangerous. But it was not evil. None of the sixteen technically are, though you may have read that Hoid has specific beef with Rayse. Whether you think of Odium as evil depends upon how much you agree with Hoid's particular view.
That said, Ruin would have been one of the 'safer' of the sixteen for Rayse to take, if he'd been about that. Odium is by its nature selfish, however, and the combination of it and Rayse makes for an entity that fears an additional power would destroy it and make it into something else.