It is possible to have multiple accounts. But the software will not allow the same account to play two characters of different nationality in the same game. We even call attention to this when you Register for the Game. However, single account players may be in the pay of the "other side" and it is possible for one player to use more than one credit card to register multiple times. Thus you have to be careful who you trust.
Spying, split loyalty between France and England, switching sides are therefore all possible. They are possible in the game mechanics and were common in the real 14th century (the real de Coucy, major French Magnate married the real Edward III's daughter and had lands in both England and France for example.
Such player activity is not cheating or dishonest. You do have to use a little judgment however. Do not say anything in your nations private message group that will get somebody killed if the other side hears of it ("I saw d'Audley at Metz, he is out of days at 6 PM PST and down to only 632 men"). Use common sense in discussing military plans. Decide based on a player-character's persona and how long you have "known" them in this game who is honest and who a crook (other game's don't count, some play radically different roles from game to game. Remember too that too much suspicion will harm you. The other side is winning the shadow war if your own defenses against espionage prevent those who DO need to know from learning, perhaps to rescue you or give you reinforcements.
The game is big and realistic enough that if you treat it with some judgment and as though it were "real" rather than a game you will do much better than if you play it as a game. Who would YOU trust at your back in a desperate fight?