What's the actual aim of the EU/NA split?
If it is just about promoting EUvsEU/NAvsNA and discouraging EUvsNA then i'm not sure a split will be enough. There's plenty of people out there for hire that mean an EU clan could attack an NA fief and hire NA fighters to do the work. Or vice versa. An EU/NA alliance could also be really overpowered as if attacking an NA clan, the EU ally can attack using the NA ally's members for the battle, whilst the NA clan being attacked can't easily attack the EU clan back without an EU ally of their own.
I can think of a few solutions to this that might help. Personally i'd like to see them all.
1) Force clans to have a max of 25% of their roster filled by non clan members when attacking. I don't think this should be applied to the defending army's roster as it would serverely hamper small clans abilities to hold onto land.
2) Severely tax the transfer of troops from 1 clan to another. I'd say this should happen anyway as it discourages silent partners in wars. Before anybody complains you will still be able to make deals for troops if you just factor the tax into the negotitions.
3) First of all, code the game so that whenever you take a fief you can pick either the server of the closest fief or the server of the 2nd closest fief to be the server of the newly captured fief. If both these 2 closest fiefs have the same server then obviously there won't be any choice in the matter. If they are different, you get a choice but once you have picked there is a 1 week cooling off period before you can change it. This makes the border between EU and NA slightly dynamic which allows the game to react to changes in NA/EU population.
However, in order to make this a less favourable option than staying on your own side of the divide I suggest doubling troop upkeep for armies that are closer to a fief that has a different server from that of the closest fief owned by the army's clan. Bit of a difficult one to explian so i'll give an example.
Say that Yalibe is EU and Reindi Castle is NA. If the EU clan wants to attack Reindi Castle then once they get closer to Reindi Castle than Yalibe, they will have to pay double upkeep. If the EU clans decides this is fine, proceeds with the attack and takes Reindi Castle then they will be able to choose which server they want to set the castle to. They get this choice because Yalibe is the 2nd closest fief and it is EU, whilst Ehlerdah is the closest fief and is NA.
If the clan decides to set Reindi to EU then the double upkeep rule will still apply if they decide to push on and try to take another NA fief. If however they decide to keep Reindi Castle as NA then they will be able to push on without worrying about double upkeep so long as Reindi remains in their possession.
This rule is complicated because it is the only way I can think of to implement any kind of increased upkeep rule without punishing clans with both NA and EU members.