I'm not sure if this only applies to when a band ability is used or if this rule transcends into our argument. If it does apply here, it would mean that once the good copy of a unique character has entered the FoB, his/her evil counterpart cannot be used in a battle that turn regardless of whether it is the same battle or not.
You are correct; the current rule says "once per turn" and not "once per phase," so regardless of how many battles are created, only one copy of a unique character can enter any battle on a single turn.
so this says that a player which means yes your opponents evil king saul could enter battle in a different battle then ambush the city even more specifically says set them aside which means that good king saul is out of play anyway.
In or out of play does not have anything to do with "control," though, just in case anyone tries to say that they can use a different copy of a unique character because it leaves play.
But who controls it is moot; no matter who controls which copies of a unique character, only one such copy can enter a single battle in any complete turn.
In the original example, the opponent's King Saul is protected by game rule from entering battle again.
A clarification should be added to the "Duplicate Cards" entry, though the wording from Banding (in place for quite awhile) is clear on the unique-battle rules.
