There's one scenario that hasnt been mentioned. What if an enemy army is sieging one of my fortresses? According to the map that province is neither mine nor his...it has both flags. It happened to me in a MP game and i expected that since moving to a friendly province happens earlier than moving to an enemy prov at turn resolution i would catch his army. After all, if that province (the one with the fortress) had both flags, it should be taken as friendly for both of us, right? Well it doesnt...he still moved before me, taking another of my provinces...and kept running...
I assume that provinces with both flags arent friendly for any of us then. But i think giving the "friendly" status for both nations would be a nice way to make fortresses more useful while defending, isnt it? I mean, raiding a fortified province would mean that you have a chance to get caught by a defending army coming for you, and should be taken in consideration both as a defensive as well as an offensive strategy.
Anyway, just a thought