In two- and three-player Citadels, how does the Bishop's ability work?
In two and three player citadels each player still only has one city so the protection of the bishop will protect it no matter your other character.  Your two characters do not directly interact, you simply get two turns and the perks for each.

For example, if you draw both the assassin and the bishop, the murder by the assassin is not undone by the bishops turn.  If you draw the bishop and the merchant, the protection of the bishop is not undone by the merchant turn.