AMC has commissioned a one-off Walking Dead special episode that apparently is a specific stand alone episode, not part of either of the shows.
The special will be set in the very early days of the outbreak and will feature characters from both TWD series together. The events take place on an airplane midflight when the outbreak takes hold on the passengers. The episode will also act as an intro for a new character who will be part of the cast for Season 2 of Fear the Walking Dead.
The special will be aired in segments online sometime during the airing of Walking Dead Season 6