While it seems as though Fire Emblem Fates could be a single release in Europe it appears as though Nintendo of America will be releasing the title as a dual release in North America as it had originally planned. For those unaware, the two versions of Fire Emblem Fates are Conquest and Birthright. The Amazon UK listing for the game suggests that the content will be on a single cartridge while the US will have to choose between which version they wish to purchase. This has been reconfirmed recently by Target reservation cards which show the two different versions. Fire Emblem Fates is coming next year here in the west.
Thanks to MasterPikachu6 for the tip