You have to destroy one hideout for Istiana and one for Arzagos respectively.
What I find bad is that you HAVE to promise both of them to stand by their cause, just to learn where the banner is. So you are inevitably forced to betray one of them, which is something that should be up to the player to decide, not dictated by the game.
Also, both Istiana and Arzagos have just told me they would not know where any more parts of the banner were after I promised them to support their cause. They have not said "one piece is in the hideout right outside of town (why does a group of 5 weak bandits have such a treasure?) but I dont know where the rest is", they just just did not inform me about that hideout in the first place.
Its pretty weak that after such long development time, not even the first hour of "guided campaigning" is done properly.