One day you can open a battle report stating that your army fought against the troops of your league mate. An answer to the question "How is it possible?" you will find in this article.
Members of one league may not raid and besiege each other’s castles. However, they can cross swords on the neutral territory, such as a Hamlet or a besieged castle of some random player.
Example #1 I send a reinforcement to the castle where my league mate had already stationed his troops keeping it besieged. Then my troops arrive at the castle and start fighting on the side of its owner liberating it from the siege of my league mate.
Example #2 My league mate stationed his troops as the reinforcement at the unclaimed Hamlet. At that, this Hamlet is still recognised as vacant because my league mate has not occupied it. I send there my battle squad and my warriors start fighting his troops.