About a third of the way through the season both time lines will be "solidified into one time" and there will be one linear time throughout the story on the island with no more flashbacks.
Both timelines? Alternate universes?
So instead of flashbacks, or flashing back and forth between Oceanic Six, are we going to flash back and forth between two alternate timelines? One where flight 815 makes it safely to its destination, and the other universe where the Oceanic Six crashed? If so, this is a major spoiler.