The Ball and the Cross by GK Chesterton

Due to certain irreconcilable differences two Scottish men, one being Catholic and the other being atheist, decide to fight a duel to the death. However, duels are against the law, so they have to find a secret place for that occasion. Searching for such place they must avoid authorities, and also numerous kinds of people who all try to convince them to give up on a duel, leading to many comic adventures.