It's obvious Locke is insane. Let's face it; it seems, considering they showed it in the "Previously on..." clip, that the emphasis on the light and dark pieces, may later represent two sides of people who come together on the island.
As I see it, Almost all people have some sort of partner.
Jack and Kate
Sun and...Guy who Orders Sun around.
Walt and his father
Sawyer and Sayed (They're enemies, but a pair)
Charlie and Claire (I think, Pregnant Lady)
Shannon and her Brother.
The Happy Happy Montage at the end of the episode showed each of these pairs bonding, and almost coming to terms with one another. Only Hurley and Locke were left out, which I think is significant.
If they really do split up into two different sides (Good and Evil, Black and White...just two different groups period) I'd expect each of these pairs to break up. The dog will run away, and Walt will blame his father. Claire will catch Charlie doing Heroin. Sawyer and Sayed don't need such a reason. The Korean wife will be tired of being opressed. Shannon will be tired of her brother's nagging.
I think Jack and Kate is the only one without some form of block easily wedged between them. And there's till 3/4 of the survivors we've yet to really meet.