[Norfleet]

The armies in question tend to end up fighting whereever is least useful for you. If you're sending two armies from two provinces to attack one province, you will always wind up with two battles: One where the enemy attacks one of your armies, and one where your other army, missing the first army, attacks the enemy province alone. There's other such examples. I'm pretty sure the game is programmed to detect such things and attempt to penalize you.