Why are there two difficulties shown when sieging towns?
Solution 1:
There is a threat level for both the enemy hero located in the entrance of the town, and additionally the hero/armies fortified within the town. Right-clicked one will show the other in parenthesis (and vice versa).
By engaging in the siege you will fight two battles at the difficulties indicated. If, however, there is no garrison hero, the garrison and the visiting hero will combine forces, and you will only fight a single battle.