This is a romantic trope where lovers share not just their lives and beds but actually their consciousness. Most often they're from an already telepathic race but this telepathy goes even further; maybe they're always aware of the other, not just when communicating but always, or they actually feel exactly like the other, or something like that. Sometimes successful reproduction is tied to it. Sometimes the sanity of one, usually the male, or both partners depends on the mindlink. Often the link is not unsoliticious. It goes both ways, so if something bad happens to one of them...