I've seen non L&C stories that have souls reincarnate in clusters. The reincarnations don't always have the same relationships but are always close. So married this time, siblings another life, best friends another, rivals etc. This can be used to explain why Lex threw away decades of caution when L&C got involved. He is part of the same cluster and that closeness made him react impulsively.
So who would you put in the same cluster with L&C? I'd say Clark's parents but not necessarily Lois's. Perry, maybe Lucy.
