This may have been discussed before, but here's my attempt to explain the "flash sideways." Technically, it's not two timelines going on at the same time considering Oceanic 815 is headed towards LAX in 2004, but the Losties are on the island in 2007. With course correction in mind, what if the new 2004 timeline in Season 6 ends where the 2007 timeline in Season 6 begins, only significantly different events occur to get them to that point. For example, Jack will "fix" Locke as opposed to Locke being magically fixed by the island. It's certainly a way to get one "true" ending.
Part of me wants it to be wrong, because it's always best when Darlton surprises you. I mean, let's be honest. How many theories did you read about how Season 6 would begin, and how wrong were %99.9 (if not all) of them?
P.S. It's like we're watching two shows right now. The only thing better than one "LOST" is two "LOST"'s.