If the combined forces of the main army + the reinforcing one(s) are less than 20 units, I'll just disable the AI and opt to control all of them myself.
If the armies combined are more than the 20 units, I'll usually select the defensive stance, and then order the army to join up at or just behind my own army, trying to delay actual combat until my armies are united. Once the actual combat breaks out, I'll then put them on aggressive stance.
That's of course if the two armies are spawning reasonably close to each other. IF they spawn on the other side of the enemy, I'd select defensive stance, and have them position themselves behind the enemy, but not close enough for actual combat, so that once the fighting starts, they can attack the enemy from behind.
No matter what you do though, be prepared for the AI to do something stupid
