Castle Bonuses: redundant or symbiotic?

[Lucky]

I am a big newbie, motivated to learn how to play by painful losses and a desire for revenge. With this as the setting, I wonder if anyone knows if the castle bonus that doubles resources in that province are tapped by neighboring castles?

Ex) 2 provinces, side by side, both have the cheapest fortifications, the tower. With their admins of 10, they steal 10% from each other. But since each's resources are doubled, would not the percentage work out to 20% in reality? Would this be bad for big provinces, giving a big 10 % to little ones, and getting a little 10% back? Would the bonuses even interact with each other? Finally, stupid as it may sound, I dream of a defensive realm, each province with it's own tower, as far as the eye could see. Would this be stupid, or prudent?

Thank you if you took the trouble to read this, and vice-versa.