[Slobby]

Once you are besieged you effectively lose control of the province to the enemy. What you could have done is have Army B move into the province and have Army A break siege and they would have combined forces.

The only exceptions that I'm aware of are a) if Army B was composed entirely of stealth units they can sneak into the castle b) if Army B teleported in they would come in through the lab (assuming one is there) bypassing the siegeing force.