Speak to Quentin, the man fightning a guard in front of the Trademond guardhouse.
After beating Quentin, Lorentz becomes available for duel in front of the Congregation building. After beating Lorentz, Reiner becomes available for duel inside the Boar's Hut (Trademond's tavern). Beat Reiner to complete the quest. For tips on how te beat each duelist, see the Duelists page.