When an army sieges an enemy settlement, it shouldn't be able to reinforce another army while still sieging. Thinking about the logistics of it, the sieging army should be preoccupied with the siege and and containment of the garrison army, so they can't be in two places at once. In order for a sieging army to reinforce a nearby battle, they must lift a siege, thereby making it possible for the enemy garrison to reinforce. There should be no other options and no difference between AI and player in this regard.
At the current state, it doesn't make sense. I am sure this was discussed in previous posts but I am hoping CA addresses this in the upcoming December patch.