Phyandarst, brother to the emperor, Knizor, has secretly orchestrated a war with another empire to gain the throne. Phyandarst is fighting on Knizor's side but is secretly coordinating the enemy attacks. Knizor's right hand dragon (all the characters are dragons) is a blacksmith named Rishta. He is the Hamilton to Knizor's Washington, the Teancum to Knizor's Moroni. Second in Knizor's circle of trust are Alkandar and Arabur, two good friends who share Rishta's rank of command. Phyandarst instigates a duel between them, then tips them off that the other dragon intends to kill them, against the rules of dueling which dictate the duel ends after first blood. (The duel is fought with swords, so death is incredibly rare as the contest ends with the first scratch, however minor.) This causes them to kill each other in self-defense. Both ask Rishta to be their second. (Knizor can't know about this.) Rishta, torn between the two, eventually chooses Alkandar. Arabur is offended, but Phyandarst, feigning sympathy, becomes his second. My question: how could Phyandarst get such two close friends so angry with each other that they would choose to settle it over a duel? Keep in mind that they enter the duel thinking their opponent will try to kill them, instead of entering the duel intending to kill their opponent. So they don't start off angry enough to kill each other over whatever it is, but angry enough to duel over it.
Pride. In a martial society, even good friends might want to show their dominance. I would feed on pride. EDIT: I should add that some foreshadowing maybe needed, so ensure their friendship is competitive.
I feel it has to be some kind of slight. Something Phyandarst sets up to get them mad at each other. Knizor is against dueling, especially since there's a war on, but something has to drive them to go against Knizor behind his back and feel justified. Rishta, on the other hand, would be conflicted not only about which friend to side with but also disobeying Knizor's orders. So what would be a way that Phyandarst could have them wound each others' pride? (Because if only one person is in the wrong, Rishta is less torn between the two.)
What about if Phyandarst killed another character, then tipped off each of Alkandar and Arabur that the other had killed him in a duel, because they refused to accept defeat?
Depending on how society is structured it may be an argument over specific loot from a battle. For example, in the Iliad, Achilles is angry with Agamemnon for taking prizes that he should not have the right to (in this case women, but doesn't have to be so in your story) so I believe an argument involving greed (made even more powerful if influenced/caused by the war they are fighting) would work quite well. An idea could be that one of the two friends have (for one reason or another - possibly based on the number of kills or what have you) believed that they deserve part of the other's prize from battle (generally these prizes are taken from a pillaged city, but take it how you please) and as a result the other friend feels wronged. It could be even better if the friend who was "wronged" originally felt that it was perfectly fair and just, but Phyandarst convinces him that he was wronged.
They're not attacking, they're defending. There are no spoils for them. What other prizes might be rewarded for a well-fought battle if the battle is a defense or reclamation of a city in the homeland?
Is it possible that it could be some sort of promotion? Could one of the two be offered more troops to command or better armor/weapons that they could agree on? I think the importance of what I stated above is that one of them is in a better situation as a result of an agreement. But after Phyandarst steps in and convinces one that he was wronged and deserves what the other has, the argument has started.
Another thing: This would be even better if this promotion or reward was given by Phyandarst's recommendation.
How does this sound? Phyandarst sets up for a general to be ambushed in battle. He warns Arabur of the danger, so Arabur will try to save him. Arabur is valiant but fails to save the general. Phyandarst tells him he should receive the general's command for his bravery. He tells Knizor the general is dead, but leaves out the part where Arabur came in. He recommends Alkandar for the position as he is a capable leader. When Arabur protests later, Phyandarst sides with him. Angered that Arabur is resentful instead of proud, he readily accepts Arabur's challenge to a duel. Rishta sides with Alkandar because he was promoted for his overall track record rather than one act of heroism, though he is still torn.
They've both married. If they leave families behind it's more tragic. Alkandar has seven children and an egg and Arabur is a widower with an adolescent daughter. And Rishta is discharged from service with the shame of enabling them when he should have prevented it. (In his defense, he wasn't expecting one of them to die, much less both.)